Gear REX

Gear Rex, also known as the Nuclear Dragon, is a monster that Snake and the MSF often encountered during Operation Peace Walker. It bears a strong resemblance to Metal Gear REX.

History
Gear Rex was created due to mutations from Nuclear Test sites. Because of the mutations, Gea Rex developed a protuberance resembling a gun turret as well as a highly acidic and corrosive mucous that is capable of melting absolutely anything it comes into contact with. However, the mucous can be reverse-engineered to form a highly flexible and sticky substance.

Gear Rex was first spotted on Isla del Monstrua, although it's most likely that it was not a native to Isla del Monstrua as there was no record of there ever being any nuclear tests in the region. Sometime afterwards, A Gear Rex originating from the Pacific entered Costa Rica from the South Pacific coast and traveled to Mt. Irazu until it reached the crater's vicinity around Noon the next day and stopped, destroying several houses and buildings in the process. Because Costa Rica had abolished it's army after the Costa Rican Civil War, it was unable to deal with Gear Rex, and thus the MSF was dispatched to stop it.

Sometime afterwards, a Gear Rex somehow managed to board the Mother Base and was causing some havoc. Because of this, the MSF had to go on the defensive and defeat Gear Rex.

Trivia

 * Gear Rex's history of being formed from Nuclear Test sites resembles that of Godzilla/Zilla.