Metal Gear Rex is a mecha which appears in Metal Gear Solid and Metal Gear Solid 4 as a pilotable mecha.
Designed by Hal Emmerich, it is armed with 30 Gatling cannon, anti-tank missiles, laser cannons and a large railgun which fires a projectile that is undetectable by any current ballistic missile detection systems and has an armor piercing warhead to penetrate bunkers. Even though it is invulnerable to conventional attacks, its large radome is vulnerable to missile attacks.
In Metal Gear Solid, Solid Snake is tricked by Liquid Snake to activate it, resulting in Solid Snake having to destroy it. After a battle, it was abandoned in the underground maintenance base.
In Metal Gear Solid 4, Rex's railgun is removed prior to Solid Snake's (now known as Old Snake) arrival in Shadow Moses island. Solid Snake pilots it and engages Metal Gear RAY, the Metal Gear designed to counter it, in battle outside the base. Due to the "Street Fighter" software control program which Hal Emmerich has secretly installed on Metal Gear Rex, Metal Gear won the battle and is abandoned after that.
