Since I only know how to model and texture using Blender 3D, and don't know squat on how to animate and export models I make into games, I have to outsource those tasks to other people in collaborated mods.
Here is a fully textured model of Metal Gear REX that would be fantastic if somebody could get it into GTA V with all of it's weapons enabled like it's anti-tank rockets, machineguns, free-electron laser, and railgun.
Just think of satisfying it would be to step on and squish people with a giant 50-foot tall, 505-ton bipedal battle tank!
Here is a link to where you can download this resource. It's a blender file, so you might have to export it into zmodeler or whatever 3D program GTA V works with.
Furthermore, here is a video from Metal Gear Solid 4 Guns Of The Patriots that demonstrates how it walks and moves.
Lot's of very complex movements.