The third in the line of Death-spewing-ass-kicking-Shaky-legged Walking tanks, in the Metal Gear Solid series. Housed in the Shadow Moses Facility, Which is taken over by FOXHOUND and it's annoyingly British Leader Liquid Snake.
The U.S Government, feeling the previous Metal Gears were a little(A lot) underwhelming decided to redisign they're Bipedal-Tank. Hal Emmerich was the man they needed and when he wasn't peeing himself in Office corners he saw fit to equip the new model with thicker armor and re-inforced Leg plating to counteract the nasty habit of enemies shooting the weak legs on previous models.
"So should we engage the enemy?"
"No, Let's just go down some stairs."
REX also has that Railgun I mentioned earlier aswell as two Vulcan 30mm Cannons and a Electron laser for when you absolutely have to kill everything in the room. REX is Sole manned and the pilot sits in the Cockpit (Hehe) shielded from danger. REX's only weakness is it's Radome, a small dish on the side that acts as it's eyes and guidance system. Seeing as MGS was released in 1998 and is set in 2005 that antenna must have had a pretty shitty reception.
Liquid:"Dammit, Somebody call the Cable guy!".
After destroying the Radome the Cockpit is left open and extremably shootable. After being destroyed by Solid Snake REX is left bent and broken on Shadow Moses for 9 years before Liquid, (in Ocelots body, consult MG series topic) seeks it out for it's Railgun. Solid Snake uses REX against Liquids Ray. Unbelievably REX wins and proves that old people and old things can be helpful and useful and shouldn't just be killed or used as paperweights.
So you've just gotten all the keys into the overide and defeated a shit load of guards, but whats this? You didn't disarm REX but armed it! Liquid is getting into REX!You only have one Ration!You my friend are Fucked!
Welcome to You're funeral! and He brought the Cake!(By Cake I mean Lasers and Machine gun fire, and yes they have Cake at funerals).
So you frantically call everyone in you're Codex, even the Guy you know is Liquid. The weapons expert is Useless, The Colonel tells you that "You must destroy Metal Gear!", Thanks for the support,Roy. The designer tells you to "aim for the Radome". So you equip the stinger, take aim and Boom. . . . . . . .Goes Snake as REX fires three Heat-seekers at him. At this point you re-evaluate you're position.
I've thought about things aaaand, i'm gonna just hid behind this tank.
When you finally take out the Radome and see the tear-jerker Grey Fox death scene you Fight REX again,Firing about 500 hundred stingers but this time is a lot easier now that the cockpit is openand you can shoot Liquid in the Face!!!.