Metal Gear REX is a CIA black project being developed at Shadow Moses during the events of the first Metal Gear Solid. It is, like other Metal Gears, a bipedal, nuclear-equipped walking battle tank. Using its railgun, it has the ability to launch a nuclear warheads from anywhere in the world. Since the railgun doesn't burn any propellant like a missile would, it won't be picked up by any ballistic missile detection systems, making it a stealth weapon.

REX's lead developer was Hal Emmerich, also known as Otacon. During development he was unaware of its nuclear capabilities, being under the belief that REX's purpose was to destroy incoming nuclear weapons.

Something I really like about REX is it's resembelance to Sahelanthropus in Metal Gear Solid V. To me, it shows how Hal is following in his fathers footsteps by continuing to make metal gears. And by giving Snake help to stop it from launching a nuke, then in the second game starting an anti-metal gear organization with Snake, he tries to rid himself of the legacy that Huey passed on as he and Snake fight for a better future.

Metal Gear REX seen in Twin Snakes (left)/Sahelanthropus in it's hunched over "Rex mode" (right)