A nuclear-equipped bipedal tank jointly developed by the US Army and ArmsTech Corporation in 2005.
It is equipped with an FCS (fire-control system) and armed with a multi-barrel machine cannon, anti-surface missiles, a laser, and a rail gun for launching nuclear warheads.
REX uses its rail gun to launch a nuclear payload at a high velocity. This eliminates the need for rocket propulsion and makes the launch invisible on radar, making it a stealth nuke. REX is the ultimate nuclear weapons platform, able to launch a nuclear strike at any location on Earth.
REX was developed by Dr. Hal Emmerich of ArmsTech. He was deceived into thinking it was a missile interceptor launch platform to be used in TMD (theater missile defense).
Hal Emmerich's fondness for robot anime led him to emphasize the bipedal aspect in his design concept.
The pilot uses the cameras on its radome to gather outside information, forcing them to open the cockpit hatch in order to see in the even that the radome has been damaged.
When Solid Snake destroyed its radome, he forced Liquid Snake to open REX's cockpit, allowing him to attack the cockpit and destroy it. REX ceased to function. It lies abandoned in the underground maintenance base on Shadow Moses Island.