Metal Gear REX

Metal Gear REX was a Metal Gear prototype that was jointly developed by DARPA and ArmsTech for the United States Army in the early 2000s.

Background
In the early 1960s, Soviet scientist Aleksandr Leonovitch Granin created designs for a REX-like bipedal tank. However, Colonel Volgin of GRU rejected his ideas in favor of Nikolai Stepanovich Sokolov's Shagohod. Drunk and bitter, Granin sent his designs to an American colleague, revealing this to CIA operative Naked Snake during Operation Snake Eater. Later, triple spy Ocelot also stole and delivered duplicate designs to his CIA superiors which later served as the inspiration for the Intercontinental Ballistic Metal Gear, the Chrysalis AI weapon peace walker and Metal Gear ZEKE in the early 1970s as well as the st-84 metal gear in 1984. The development of REX itself was foreseen in a psychic premonition by Elisa and Ursula during the San Hieronymo Incident in 1970.

Some time after Solid Snake's encounters with TX-55 Metal Gear in 1995 and Metal Gear D in 1999, an attempt to carry out a Metal Gear project within the United States had been planned but was officially scrapped yet convinced the private weapons company ArmsTech that building one for the U.S. Army would be a very profitable venture. ArmsTech had been in a dire state financially, with the possibility of a hostile takeover becoming apparent after failing to obtain a contract on a new line of fighter jet for the U.S. Air Force. Likewise, the Department of Defense agreed to its development due to the defense budget being slashed by 15% and thus being forced to allow the private sector to develop their weapons for them.

Development
In 2002, DARPA Chief Donald Anderson was heavily bribed by ArmsTech to back the Metal Gear REX project. For several years, regular payments in the tens of thousands of dollars were made to the corporate account of a dummy company for which his wife ostensibly acted as a consultant. DARPA funded REX's development covertly, using part of the United States' black budget. After the cancellation of the Arsenal Ship Project in 1996 following the death of the 25th Chief of Naval Operations, the budget had freed up, allowing the REX project to proceed undelayed. Though the CNO's death was officially ruled as suicide, rumors to the contrary circulated at the time.

The nuclear weapons disposal facility on Shadow Moses Island was constructed to conceal the development of Metal Gear REX. Its rail gun was based on technology developed for the SDI, having been successfully miniaturized in a joint venture between ArmsTech and Livermore National Labs. Livermore Labs also developed a new type of nuclear weapon for REX using NOVA and NIF laser nuclear fusion testing equipment and supercomputers.

One of the main goals of developing REX was to overcome a global trend in military downsizing at the time. A weapon such as REX would cause an immediate rift in the global power structure. REX was a self-mobile launch platform capable of using its massive rail gun to launch a nuclear warhead contained projectile package with a low radar cross-section, and without the need for a rocket propulsion system, the launch event was also undetectable by satellite thermal imaging unlike the massive heat signature characteristic of ICBM launches. It could give any country in the world a first-strike capability that other nations would find hard to counter. This also exploited a specific loophole that existed in anti-nuke treaties, since projectiles fired from a rail gun were not technically ICBMs, so the START treaties imposing limitations on ballistic missiles did not apply.

During REX's development, the Metal Gear engineers became concerned of what would happen if REX, after it was completed and doing solo ops in the field, was detected by the enemy and forced to engage in close range combat against conventional forces. They then decided that they could make REX's hull a weapon in itself. Although the program was completed, and had gotten fantastic results from supercomputer simulation, the military shelved the project before they could even install it (as the military viewed it as not being part of their regulations). However, REX's chief engineer Hal Emmerich secretly uploaded the data into REX anyway. Although the first actual prototype for REX was developed in 2005, it went through multiple prototype stages in VR while it was still in the design stage to correct various flaws before being developed. As REX was primarily tested via VR simulations, the Pentagon and ArmsTech needed to perform live testing to compare the simulation data with actual test results. Because of this, they started a field exercise at the remote military facility on Shadow Moses Island.Metal Gear Solid, Konami Computer Entertainment Japan (1998) Otacon: Snake, I did it! // Solid Snake: You got past security? // Otacon: Bingo!!! // Solid Snake: Great. So whaddya got? // Otacon: I accessed the confidential Metal Gear file. // Solid Snake: So what about the PAL override system that Baker talked about? // Otacon: I haven't found it yet. // Solid Snake: That's what I need to know! // Otacon: But Snake, I found something else! // Solid Snake: What? // Otacon: The secret behind the new nuclear weapon! Just as I thought, the nuclear warhead is designed to be fired from the rail gun, like a projectile... It doesn't use fuel so it isn't considered a missile. That way it can get around all sorts of international treaties. // Solid Snake: Pretty sneaky. // Otacon: Yes, but effective. And that's not even the scariest thing about this weapon. // Solid Snake: I can't wait to hear this. // Otacon: It's a stealth weapon! // Solid Snake: You mean it won't show up on radar? // Otacon: Yeah. The truth is... they've been working on a stealth missile since the late 70's. // Solid Snake: Why weren't they able to develop one until now? // Otacon: Because of the missile's rocket propulsion system... it will be picked up by enemy satellites. // Solid Snake: Oh yeah, that makes sense. // Otacon: But unlike a missile, the rail gun doesn't burn any propellant. So it can't be detected by any current ballistic missile detection systems. // Solid Snake: An invisible nuclear warhead... // Otacon: Totally impossible to intercept. And on top of that, it's got a surface piercing warhead designed to penetrate hardened underground bases. // Solid Snake: We learned that lesson in the Gulf War. // Otacon: This thing could mean the end of the world! // Solid Snake: It's the ultimate weapon. And from a political point of view, it avoids the problem of nuclear reduction and nuclear inspections. Colonel, is this true? Are you listening? // Campbell: I'm listening... // Solid Snake: If word of this got out, it could delay the signing of the Start 3 Treaty and cause a huge international incident. // Otacon: Yeah, it would be nasty. The United States would be denounced by the UN... it could even bring the President down... // Solid Snake: Did you know this, Colonel? // Campbell: I'm sorry... // Solid Snake: You've changed, Colonel. // Campbell: I won't make any excuses... // Otacon: Snake, listen to me. This new nuclear weapon... it's never actually been tested, only simulated. // Solid Snake: You mean they ran a computer model? // Otacon: Yeah, that's why they were conducting this exercise. They needed to get actual data to back up the simulation. // Solid Snake: What were the results of the exercise? // Otacon: It looks like it went better than they hoped for. But...I can't find the data anywhere on this network. You'd think that data as important as that would be carefully recorded... // Solid Snake: It was. President Baker gave me an optical disc with all of the test data. // Campbell: What!? Do you still have it? // Solid Snake: No, Ocelot took it from me. // Campbell: Damn... // Otacon: The terrorists have replaced the dummy warhead with a real warhead. Once they input the detonation codes, they should be ready to launch. // Solid Snake: So you think they can do it? // Otacon: Well the dummy warhead was designed to be identical to the real thing, so I think so. // Solid Snake: Did you find out how to override it yet? // Otacon: Not yet. It must be in a separate file. Right now, I'm looking through all of Baker's personal files. // Solid Snake: We're counting on you...

In 2005, both ArmsTech President Kenneth Baker and DARPA chief Donald Anderson attended a rail gun testing exercise for Metal Gear REX at the nuclear weapons disposal facility on Shadow Moses Island. However, REX's prototype was captured by the special forces group FOXHOUND, which had originally been assigned to oversee the final testing of the secret new weapon. The threat of a nuclear attack on the White House was used in an attempt to extort Big Boss's remains from the U.S. Government and later one billion dollars in cash. The Pentagon wanted to both stop the terrorist threat against them and recover REX without risking damage so sent Solid Snake to use as a vector for FOXDIE. Baker himself was also targeted in an attempt to silence him regarding REX's development, affecting him just as he implied REX's ability to fire stealth nukes from its rail gun when telling him to quickly stop the terrorists before the incident becomes public and resulted in him and his company being "finished."

Hal Emmerich was opposed to nuclear weaponry, and it was unlikely he would have assisted in the program if he had known REX's true purpose. He was informed that REX was intended as an anti-ballistic missile system for intercepting and destroying incoming nuclear weapons, and REX's primary armament was handled by a separate department to keep him in the dark. Their development and integration of the weapon systems into the main chassis were out of his control, and as far as he knew, these armaments were intended to defend the REX unit itself, and to provide Theater Missile Defense.

It was only during the Shadow Moses Incident that Otacon discovered REX's true purpose as a nuclear launch platform, and its revolutionary delivery system – a rail gun. As the projectile it fired was not actually a missile, it wasn't covered by current disarmament treaties. Even worse, the combination of the size, speed and nature of the projectile could allow the payload to bypass current missile detection systems, which had always been dependent on the characteristics of missiles – size, speed, and thermal exhaust signature – to track them. In addition, the warheads themselves were also stealth-equipped, so as to not appear on any of the current radar systems.

REX was the ultimate Metal Gear, as it was not only capable of delivering a nuclear weapon both to and from any point on the globe, it could do so "undetectably" and "untraceably." All previous Metal Gears were merely mobile ground-based launch systems for nuclear missiles, little more than the land-based equivalent of ballistic missile submarines. REX utilized a rail gun, which fired much smaller projectiles without chemical propellants and contained stealth technology – factors which made them virtually invisible to radar. It thus fatally undermined the 'no first strike' principle of Mutually Assured Destruction – whoever controlled it could launch a nuclear attack at any target at any time, with no warning prior to impact and detonation. In addition, its missiles were designed have sufficient penetration to destroy underground bunkers, a lesson that was learned from the Gulf War.

By this time, the personnel had managed to replace the test dummy warhead with a real one (the dummy was designed to have identical characteristics as a real one, in terms of external aerodynamic profile and mass distribution). It became clear that REX was in fact to be used to make an actual nuclear strike; Otacon took it as his duty as REX's creator to destroy it. Liquid Snake and Revolver Ocelot later intended to join forces with former-Spetsnaz colonel Sergei Gurlukovich with REX on their side.

After Snake was unwittingly deceived into activating REX via the PAL system, he attempted to stop the machine by direct means. By that time, Liquid was piloting the weapon when Solid Snake encountered it, and attempted to kill him with REX's conventional weapons. Snake, using Otacon's tactical advice, managed to damage REX’s radome using Stinger missiles, in an attempt to blind the pilot due to the fully enclosed cockpit. After several successful hits, the machine was temporarily immobilized. However, the missiles caused less damage than Snake expected, and REX became fully operational once more, attempting to crush Snake beneath one of its feet. Before this could happen, Gray Fox intervened, using the cybernetically-enhanced strength of his Cyborg Ninja exoskeleton to hold up REX's foot and thwart its attack. Fox then damaged REX's radome further, with several blasts from his exoskeleton's gun and his sword, allowing Snake and Fox a temporary respite. Providing Snake with additional missiles, Fox attempted to halt REX's advance, but his left arm was dismembered by REX's laser, and he was eventually pinned against a wall by its beak-like cockpit. In an act of sacrifice, he got off a few more shots from his gun, succeeding in destroying the radome completely, and forcing Liquid to open the cockpit to see, though Fox was killed beneath REX's trampling feet shortly afterward.

Now having the chance to finish off REX, Snake managed to launch a Stinger at its now-vulnerable open cockpit. As Liquid proceeded to step on Snake with REX once more, the machine suddenly began to lose its balance, the control systems having become inoperable from the cockpit damage sustained, and the resulting explosion knocking Snake unconscious. Still standing strong, Liquid dragged Snake to the top of REX's head, where the two engaged in a bare handed fight. At the end of the deadly battle between the two, Snake's finishing blow knocked Liquid off the top of the Metal Gear, though he would survive the fall to the ground below.

Post-Shadow Moses
The Shadow Moses Incident placed the plans for REX's mass production on indefinite hold, and the entire development was scrapped due to the loss of the REX development and engineering data, which included solutions necessary to overcome design issues with the rail gun nuclear delivery system. Ocelot ended up supplying REX's stolen data to Solidus Snake following the events of the incident.

Sometime afterwards, Ocelot leaked the Metal Gear exercise data on to the black market, unleashing a swarm of Metal Gear derivatives throughout the world (and also made enough money to buy a decent-sized nation). In response, Snake and Otacon formed the anti-Metal Gear NGO Philanthropy and sabotaged as many Metal Gear development operations as they could while the U.S. Marines's response was to develop the new Metal Gear hunter-killer Metal Gear RAY to eliminate REX derivatives.

In the meantime, the damaged wreck of the original prototype lay wasted and abandoned in the underground maintenance base where it had been destroyed.

In 2014, Liquid Ocelot returned to Shadow Moses Island and salvaged the rail gun from Metal Gear REX's remains with which to launch a nuclear strike on the Patriots's core AI. Since it had been developed before the SOP system's introduction and had lain untouched since 2005, it was the only WMD that could be used to make such a strike. Old Snake (Solid Snake) returned to Shadow Moses himself in an attempt to stop this plan though the rail gun had already been removed by the time he reached the underground hangar. He instead fought Vamp until Raiden took over for him, after which he fended off waves of Suicide Gekko sent to destroy the hangar. Meanwhile, Otacon set about restoring power and functionality to REX, via the Metal Gear Mk. III, as Raiden and Vamp fought atop the Metal Gear. Shortly after Vamp's defeat, Otacon succeeded in reactivating REX, and used its still-functional anti-tank missiles to block the passage of more oncoming Gekko.

With REX now marginally operational, Snake and Raiden climbed aboard, with the former taking the controls to pilot it out of the hangar. Raiden abandoned REX shortly thereafter to obstruct the Suicide Gekko's pursuit, while Snake fought through the continuing onslaught using REX's armaments. REX eventually reached the surface, arriving at the island's supply port. It was there that Snake was confronted by Metal Gear RAY manned by Liquid. Increasing REX's performance through the supercomputer onboard the Nomad, Otacon activated the Metal Gear's "Street Fighter" program, putting it on a more equal footing with RAY in terms of combat abilities. The two Metal Gears then clashed, and despite being handicapped by its inoperable radome, in Snake's hands, REX succeeded in holding its own against RAY. The two were eventually rendered inoperable after sustaining heavy damage, and Liquid made his retreat on foot towards his waiting ship. The impact shocks sustained by REX's frame during the battle caused Snake to injure his shoulder, and he was unable to accurately fire on the fleeing Liquid. Failing to finish off his wounded opponent thanks to Raiden and the USS Missouri, Liquid made his escape, and following Snake's subsequent rescue, REX was once again abandoned on the island.

Name
Metal Gear REX is named after the Tyrannosaurus rex, due to its theropod-like design; other similarities include the open cockpit's resemblance to a gaping mouth, along with its ability to roar. The word Rex itself is Latin for "King," which was also the Allied codename used for the Japanese Kawanishi N1K floatplane during World War II.

Operation
Metal Gear REX is operated by a single pilot within a completely self-enclosed cockpit, shut off from the outside environment. The radome on REX's left arm houses multiple electronic sensors that are connected to a high tech virtual reality interface within the pilot's seat. Destruction of the radome, however, cuts off the sensory data feed and renders the VR interface inoperative, forcing the pilot to manually operate REX by opening the cockpit, which also exposed it to enemy fire. This flaw was deliberately engineered by Otacon during development, as he felt it wouldn't truly be complete without a weakness or "character flaw" of some sort.

Armament
REX differed from earlier Metal Gear models in that its legs were heavily armored and reinforced; not vulnerable like its predecessors. Along with near-impenetrable compound armor (of which only HEAT weapons could deal sufficient damage), REX also had a pair of 30mm, XGAU-8R rotary cannons, AGM-114P anti-tank missiles, and a free-electron variant of the Mobile Tactical High-Energy Laser (MTHEL-FE) to protect itself from conventional forces. Its feet also possessed retractable metal stake-like protrusions, which were used to find purchase on difficult terrain, and could be employed in "stomp" attacks on an enemy.

The AGM-114P anti-tank missiles, launched from both of REX's knees and back, were a laser semi-active homing type that didn't use wires. The launchers used a low-powered laser illuminator to bounce a coded laser beam off a target that the missile would then home in on. The free-electron laser cannon mounted on REX's belly was capable of generating almost 100 megawatts of energy, ten times greater than any other laser at that time.

REX's most fearsome weapon, however, was the magnetic rail gun capable of delivering an untraceable nuclear warhead anywhere in the world, without the propellant trail or launch flare that gives away the launch position of a traditional ballistic missile. By perfecting the process of electromagnetic acceleration, the rail gun is able to fire a projectile with a muzzle velocity of over 100 kilometers per second. It was apparently originally going to be used, as part of SDI, to shoot down enemy ICBMs outside the atmosphere. Otacon stated that when fired outside the atmosphere, REX's nuclear rounds would have a radius of effect of 300 meters, and the gun would be sufficiently accurate to hit a satellite traveling at 10 kilometers per second. The rail gun's feature was also implied by ArmsTech President Kenneth Baker to be illegal, and that were it to be discovered, his company would most likely end up shut down and Baker himself would most likely face an inquiry.

During Liquid Ocelot's Insurrection, Otacon revealed to Old Snake that he had installed a "Street Fighter" software control program into REX that gave it not only an agility boost, but enabled the Metal Gear to execute kicking and other melee moves, which would be lethal to many military vehicles and other Metal Gears such as Metal Gear RAY. This program was one of REX's most lethal weapons compared to its other conventional warfare weapons, as military vehicles and Metal Gears were designed with countermeasures against conventional armaments, but not the raw brute force of these attacks.

Unconfirmed history
Prior to the terrorist takeover, Metal Gear REX underwent three test launches of a dummy warhead, all of which were contained in Kenneth Baker's MO disc. REX required four tests before it could be utilized within conflict, of which the Department of Defense would oversee the first three tests, while the CIA would control any subsequent tests.

Hal Emmerich's reason for giving it the codename "REX" was to reference Jurassic Park, an idea that even he felt was silly in retrospect.

Metal Gear Solid
The first Metal Gear to appear in 3D, REX's appearance (with numerous flat surfaces and little exposed wiring) was tailored with the graphical limitations of the PlayStation console in mind. In Metal Gear Solid, the codename is rendered "Rex," while subsequent games and sources render the whole name in capital letters.

The Metal Gear Solid: Official Mission Handbook gave a list of specifications regarding Metal Gear REX. However, details regarding the machine guns and air ordinance are contradicted by statements made in Metal Gear Solid about these weapons. Because of this, the canonicity of this information in question:

METAL GEAR REX - STANDARD MECHA DEFENSE MACHINE WALKING BATTLE TANK (WBT) - REX0023b ''' Crew: 1 (nose cockpit) [Armament] Machine-guns: 2xg12.7mm[sic] in the nose (covering 60° arc to the front) Air ordinance: 3xPhalanga-F (AT-3 "Rain from Heaven") Radio-guided anti-troop missiles Rail Gun: 18.5m Rail Gun (Special Ops Optimum Bombardment 422 "Widowmaker"), launch tube: Standard (conventional, nuclear) Laser: ArmsTech International V17 Vulcan Cannon, Searing LaserStorm High Energy Cutter "Slice n' Dice" (covering 90° arc) Ammunition: 10,000x12.7mm, 72xPhalanga-F Reds, Laser Pro Pack, 1xCruise AGM-129 ACM Length (gun forward): 18.9m Length (hull): 7.455m Width: 5.760m Height (gun forward): 12m Height (gun up): 13m Weight (combat): 505,450kg Power-to-weight Ratio: 450hp/ton Engine: ArmsTech AGM 4000 "Behemoth" Maximum Road Speed: 140.777km/h Maximum Range: 1,505km Fuel Capacity: 5,550lit Armor (type): Laminate/Steel NBS System: Yes Night Vision Equipment: Yes''
 * ' ARMSTECH LAND SYSTEMS DEPARTMENT

The method by which REX kills Gray Fox is portrayed differently between the various versions of Metal Gear Solid. In the original PlayStation version, REX stomps on Gray Fox twice, the second of which crushes him to death. In the remake Metal Gear Solid: The Twin Snakes, REX only uses one stomp to kill Fox, gradually applying greater pressure until his exoskeleton eventually gives way. In the novelization, while REX does briefly step on Fox, he isn't killed immediately as he manages to destroy its radome with his gun before it can completely crush him.

In The Twin Snakes, REX shares the same boss BGM theme as the Hind D, similar to Metal Gear 2: Solid Snake with the Hind D and Metal Gear D.

Metal Gear Solid 2: Sons of Liberty
According to Hideo Kojima's "Grand Game Plan" for Metal Gear Solid 2: Sons of Liberty, some of the countries that Ocelot leaked the REX data to were the five big nuclear powers other than the United States, as well as Iraq and North Korea. In addition, Solidus was explicitly stated to have been involved in the leaking, and he only supplied Iraq and North Korea with the REX data because he knew full well that they wouldn't be able to create it, anyway.

Within the game itself, REX appears during various flashbacks to the Shadow Moses Incident, utilizing footage from Metal Gear Solid.

Metal Gear Solid 3: Snake Eater
Metal Gear REX makes a cameo appearance in Metal Gear Solid 3: Snake Eater, inside the Graniny Gorki facility. During the cutscene in which Granin shows Naked Snake his Metal Gear documents, the player can enter First Person View and see the designs for REX. A screenshot image of REX from Metal Gear Solid can also be seen mounted on the walls in various locations throughout the building.

Metal Gear Solid Mobile
Metal Gear REX also appears in Metal Gear Solid Mobile, although it was implied to be one of the REX derivatives. The REX unit, however, was later revealed to be part of a VR simulation by the Patriots.

Metal Gear Solid 4: Guns of the Patriots
Metal Gear REX is operated by the player at the end of Act 4 in Metal Gear Solid 4: Guns of the Patriots. REX is the first Metal Gear to be playable in the series (not counting the Metal Gear Mk. II and Mk. III). While piloting REX, pressing the Triangle button will cause REX to "roar." Despite Otacon stating that REX's anti-tank missiles were laser-guided in Metal Gear Solid, and Metal Gear Solid 4 listing the missiles' model as the laser-guided AGM-114P, the "AT missile" weapon description refers to them as radio-guided.

Though it was REX's left foot that killed Gray Fox in Metal Gear Solid, taking a photograph of its right foot will cause Fox's ghost to appear in the image.

The battle damaged REX in  Metal Gear Solid 4'  has its left-hand gatling gun missing its cover. While the gun is stated to be an XGAU-8R (instead of a "Vulcan" as in Metal Gear Solid), the gun shown only has six barrels; the actual GAU-8 has seven.

In Metal Gear Solid 4, during any mission briefing, a small model of REX can be seen besides Otacon's computer.

The game and the novel have differing explanations for how REX was able to beat RAY. In the game, it was stated to be because Otacon had secretly uploaded a program into REX during its development that the military did not approve of. In the novel, he implies that it was largely because the RAY Ocelot was piloting was reverse-engineered from an AI-controlled weapon to a manned vehicle, which left it at a disadvantage to REX, which was intended to use a human pilot from the start.

Super Smash Bros. Brawl
Metal Gear REX appears alongside Metal Gear RAY and Metal Gear Gekko on the Shadow Moses Island stage in Super Smash Bros. Brawl.

It also appears as a sticker (allowing the player to start a battle with the Cracker Launcher) and trophy with the following description:

"A war machine of ruinous power. Metal Gear REX was developed at the weapons company ArmsTech under the eye of Hal 'Otacon' Emmerich. It had the ability to launch nuclear warheads from a mounted rail gun. During testing, REX was captured by FOXHOUND, a rogue special-forces unit headed by Liquid Snake, but Solid Snake managed to destroy the machine before calamity ensued."

Metal Gear Solid: Peace Walker
In Metal Gear Solid: Peace Walker, a secret boss can be encountered on Monster Island called "Gear REX," which has several striking similarities to Metal Gear REX, such as bone ridges that make its face look like the Metal Gear, and a limb over its right shoulder representing the rail gun, which shoots a highly acidic mucous instead. Its roar also sounds similar to that made by its namesake.

Although Otacon talks about rail gun technology having been successfully miniaturized for REX in the original Metal Gear Solid, a similar-sized weapon appears on the Chrysalis AI weapon and Metal Gear ZEKE despite this, albeit with conventional ammunition and a somewhat more archaic look.

Other
Metal Gear REX made a cameo in Metal Gear Solid: Portable Ops Plus as part of a playable online map based on Shadow Moses Island's Underground Maintenance Base called "REX Hangar."

Metal Gear REX is a game card for the Risk board game Risk: Metal Gear Solid. Players who manage to obtain REX and the PAL Key will be able to have long range options and thus launch a nuclear strike on enemy territory from far away.

A 1/48th scale figure of Rex was developed by ThreeA. It was designed by Ashley Wood with direction by Yoji Shinkawa for $490 USD, and has already been sold out. It was on sale for pre-order on March 6th, 2012. It has light up capabilities ranging from its Railgun to its pilot seat. ThreeA also made much of its armor removeable so it could be displayed with battle damage, as seen in Metal Gear Solid 4. People who pre-ordered from 3A's online store Bambaland got an additional poster and a figure of Old Snake as the pilot.

A 1/100 scale unpainted plastic snapfit model kit of REX was released by kotobukiya, it was released for sale on December 28, 2012. The kit came with small figures for Snake with the Stinger Launcher, Liquid as the pilot, and Gray Fox standing and being crushed to set up action sequences.

Metal Gear REX appeared as a participant in the Konami-sponsored event E3 Battle, where it defeated Bahamut from ''Hard Corps. Uprising'' for the first round, its weapons systems standing up to the one-man Hard Corps. army. It then faced off against and defeated Heather Mason in the second round. It then fought Zobek in the third round and defeated him. It later proceeded to finish off Sniper Wolf in the fourth round. It then faced off against and defeated The Boss in the semifinals. It will proceed to fight against Naked Snake in a no-holds barred beatdown for the finals. Despite this, however, REX lost to Naked Snake, prompting the latter to become champion.

"METAL GEAR REX FIRST APPEARED IN Metal Gear Solid (1998) A bi-pedal, nuclear-armed tank. Developed by DARPA and ArmsTech."

- Metal Gear REX's bio from E3 Battle

A LEGO-based model of Metal Gear REX was eventually created, with the Metal Gear Solid Facebook page unveiling it and crediting its creator, ragnarock1.

It later appeared in Versus Battle, where it faced RAY and later against Big Boss. However, in both its appearances, its bio was different.

"STRENGTHS: Heavy Artillery WEAKNESSES: Water FEATURED FACT: METAL GEAR REX’s rail gun was an evolution of the technology used for the SDI program FEATURED GAME: METAL GEAR SOLID 4: Guns of the Patriots"

- REX's data from Versus Battle.

"STRENGTHS: The Terrifying T-Rex of METAL GEARS WEAKNESSES: Destroyed Radome = Open Cockpit FEATURED FACT: Cannons, Missiles, & Lasers, oh my! REX’s arsenal makes it a king of battlefields, but its killer weapon is its lightning-fast nuclear-launching rail gun FEATURED GAME: METAL GEAR SOLID: THE LEGACY COLLECTION"

- Metal Gear REX's bio on Versus Battle

REX also appears as an unlockable Metal Gear model for the Mother Base developer in the Ground Zeroes app. It is unlocked by obtaining a lethal headshot from less then 398m within the main game while the app is connected to it.

Although REX itself doesn't appear in the third iteration of Metal Gear Online, its likeness is utilized as a special DLC helmet for the various soldiers. It is included in the Collector's Edition and Premium Package of The Phantom Pain for MGO-specific DLC.

Appearances

 * Metal Gear Solid (first appearance)
 * Metal Gear Solid: The Twin Snakes
 * Metal Gear Solid 3: Snake Eater (designs)
 * Metal Gear Solid: Portable Ops (prophetic vision)
 * Metal Gear Solid: Portable Ops Plus (online map)
 * Metal Gear Solid 4: Guns of the Patriots