Metal Gear Sahelanthropus

In 1984, a progenitor model to the later Metal Gear REX was developed. Soviet in origin, Huey Emmerich was suspected to have helped develop it. In neutral position, it stood approximately 15 meters tall.

History
Some time in 1984, Eli stole and took command of the Metal Gear to chase down Big Boss, but was stopped by a narrow gap in the canyon it was running in. Diamond Dogs had somehow managed to retrieve the Metal Gear and place it at Mother Base.

Behind the scenes
This vehicle is currently named ST-84. It was first revealed in the E3 2014 trailer of Metal Gear Solid V: The Phantom Pain. More details of it were given in the E3 2015 trailer and the Japanese special edition of The Phantom Pain.

It's possible that the REX prototype is actually the Metal Gear concept that Aleksandr Leonovitch Granin showed to Naked Snake, and later ended up in the hands of Huey.