In Metal Gear Solid, the nuclear-armed walking tank developed by ArmsTech is Metal Gear REX.
REX is a bipedal nuclear weapons platform secretly developed on Shadow Moses Island. ArmsTech designed it for the U.S. military, while Hal Emmerich worked on its engineering before discovering that the project could launch nuclear weapons with little warning. The machine combines heavy armor, a rail gun, and a single-legged design intended to operate over difficult terrain.
Solid Snake first sees REX during the Shadow Moses incident and later battles it after Liquid Snake takes control. The game’s story distinguishes REX from earlier Metal Gear models, which were generally presented as mobile nuclear-launch platforms rather than this particular prototype.
Players sometimes confuse REX with RAY, the amphibious machine introduced in Metal Gear Solid 2, or with Shagohod, the large Soviet vehicle from Metal Gear Solid 3. REX is specifically the ArmsTech prototype central to the original Metal Gear Solid.