From the Metal Gear Wiki

Metal Gear refers to the first ever Metal Gear model, and Metal Gear RAXA (pronounced rah-sha) was it's prototype intended for performance evaluation. This model of Metal Gear was developed by both Sokolov and Granin.

Chronologically, this was the first Metal Gear prototype ever built, having been developed by the U.S. government in 1970. First seen during the San Hyeronimo Incident, its design, completed by Sokolov, is based on the original blueprints created by Granin. The head itself has a strong resemblance with the one from Metal Gear REX, a later Metal Gear model from the Shadow Moses Incident. Metal Gear has four legs, which was merely a step towards bipedalism. Its nuclear function is to act as a mobile launching device for MIRVs. While therefore capable of making nuclear strikes against several targets at once, its range is limited, and unlike the Shagohod, is unable to compensate for it with speed. Thus, Metal Gear must be physically transported to a point within range of the target(s) first. This is accomplished by having the Metal Gear unit itself attached to the top of a rocket, launching it, detach the unit at 3000 ft above the intended landing point, and having it parachute back down to the ground, launching its nuclear payload afterwards. An experimental prototype model, code-named RAXA (outwardly identical to the final unit), was produced beforehand, intended for performance evaluation.

[edit] Big Boss’ Interference

Having been misled to believe it to be the complete (and only) model at the time, Big Boss intended to sabotage it before it could be activated and used to deploy nuclear strikes. Once he arrived however, it instantly became active, being piloted by Colonel Skowronski, who had snuck himself in with the intention of using it against Gene. Very soon after its activation, its incompleteness became quite apparent, and it suddenly shut itself down while firing its machinegun. At Gene's command, Ursula awakened and commandeered RAXA herself, using her psychic powers not only to compensate for the malfunctions, but to enhance its performance beyond its normal standards, causing it to move unlike a machine at all. Colonel Skowronski is thrown off the cockpit and falls to his death. Big Boss is forced to fight Metal Gear along with his comrades, by first targeting and disabling its four legs, but then it demonstrated an impressive capability for functioning even without them, using its "wings" and hover propulsion engines to drag itself about. Regardless of this, Big Boss still managed to destroy it, by attacking its missile tubes when they opened up.

After enjoying a brief sense of victory, Big Boss is informed that what he just defeated was merely the test model, and then witnessed the real unit being transported via helicopter toward the base's underground silo complex. Ultimately, he is unable to reach the launch control room before Gene can begin the launch countdown, and must resort to trying to destroy the unit itself. While it was fortunately unmanned and already attached to the top of the rocket (and thereby effectively defenceless) it was, however, said to be invulnerable to conventional weaponry, thanks to its incredibly thick armour, and supposedly any assault on the rocket could cause the underground silo complex itself to be destroyed in the resulting explosion. Determined to save the world without any concern for his personal safety, Big Boss commits himself and attacks it with his RPG-7, while the troops he had inspired over the course of the incident helped him out by firing their own weapons. Although the rocket launched anyway, it quickly became clear that all the combined firepower damaged it sufficiently: After detaching from the rocket, Metal Gear began to fall off course, resulting in its destruction once it crashed back down to the ground.

Metal Gears
Main Series
Shagohod — Metal Gear RAXA — Metal Gear TX-55 — Metal Gear D — Metal Gear REX Metal Gear RAY — Arsenal Gear — Metal Gear Gekko — Metal Gear Mk. II
Non-Canon Metal Gears
Metal Gear Chaioth Ha Qadesh — Metal Gear GANDER — Metal Gear KODOQUE