A Kenneth Baker/Armstech industries black project, Metal Gear Rex is a bipedal mech defense vehicle covertly developed by chief engineer Hal "Otacon" Emmerich and funded by DARPA chief Donald Anderson. Although Emmerich was assured that Rex would be used only for nuclear deterrence, a separate department within Armstech had far different plans: Metal Gear Rex would be the world's first nuclear capable tank, able to strike from anywhere on land.
Beyond its nuclear armament, Metal Gear Rex possesses a vast array of powerful weaponry and defensive attributes. Offensively, Rex holds two G12.7 mm Machine Guns, three Phalanga-F (AT-3 "Rain From Heaven") Radio-Guided Anti-Troop Missile Launchers, one 18.5m Rail Gun (Special Ops Optimum Bombardment 422 Widowmaker), and one Armstech International V17 Vulcan Cannon Searing LaserStorm High Energy Cutter. Defensively, the vehicle is encased in a virtually impregnable laminate / steel alloy.
Rex's lone pilot is completely dependent on a radome mounted on the mech's left shoulder to provide information about the outside world and auto-target the weapons systems. If the radome is destroyed, the pilot is forced to manually control Rex's offensive and defensive capabilities, subsequently becoming much more vulnerable to enemy fire.
Shadow Moses Incident
The Incident at Shadow Moses started when the Next-Generation Special and FOXHOUND went rogue and took over the nuclear facility in which Metal Gear Rex was being held. Their plan was to shoot a nuclear missile if the United States did not give the terrorists the remains of Big Boss.During the Shadow Moses Incident, Solid Snake was charged with the finding and destroying of Metal Gear Rex. However, when he eventually able to locate the armored tank, Liquid Snake entered the cockpit and started to attack Solid Snake. Fortunately, an engineer who who worked on Metal Gear Rex told Snake that in order to defeat Metal Gear Rex and Liquid, he would need to destroy the radome, which who force Liquid to open the cockpit and manually use Metal gear Rex. Eventually, Solid Snake was able to defeat Liquid Snake and destroy metal Gear Rex.
Post Shadow Moses
After the Incident at Shadow Moses, one of the members of FOXHOUND who was at Shadow Moses took data from Metal Gear Rex and sold to the black market, which in turn created many Metal Gears. As a result, Solid Snake and Otacon created a group called Philanthropy which was an anti-Metal Gear group with purpose of exposing many Metal Gears to the world and eventually destroying them. During all of this time, the original Metal Gear Rex was still at Shadow Moses was to was destroyed years ago.
Liquid Ocelot's Insurrection
In 2014, Liquid Ocelot's plan was to take total control of The Patriots and lead the world into an international war. In order for this to work, Ocelot had to destroy JD (John Doe) the main computer AI that controls the Patriots. However, JD was disguised as space debris and Ocelot had no nuclear missiles under his control. However, he was able to use Metal Gear Rex because it was not under the control of The Patriots since it was created before The Patriots started to register weapons including Metal Gears into its system. However, Ocelot only needed Rex's Rail Gun to shoot a nuclear missile at JD.
When Solid Snake came back to Shadow Moses, he had to pilot Metal Gear due to the fact that the nuclear facility on the island was going to explode. However, he was able to make it outside Ocelot showed up with Metal Gear RAY (the successor of Metal Gear Rex). As a result, both Metal Gears fought. Solid Snake and Rex won the fight against RAY with the assistance of Otacon with the end result of RAY being completely destroyed and Rex shutting down for good.