The X-O Powersuit was the first bipedal walker designed by GDI. It is commonly considered the grandfather of walkers used in the Second Tiberium War.
Background
The suit was roughly 1.5 times the size of a human (10') and had two articulated arms with weapons and mechanical hands attached to them. The main body stood on two armoured legs and was shaped roughly like an egg with a large, armoured glass cover on the top, where the pilot was located. It weighed at about 2000 lbs.
The suit's main purpose was covert operations requiring quick and effective strikes with full anti-armor potential. To this end, the walker had a high powered 20 MW laser, an 8mm minigun, and a 35mm rocket launcher equipped. The two legs gave it great speed and allowed it to track down enemies quickly. It also carried video and IR imaging equipment in armoured components on its back.
Suits could also be equipped with jetpacks for airborne deployment and attacks. It is often referred to as the predescessor of the Wolverine.
Promo render
Ditto
Concept art
Ditto
Ditto, jetpack variant
Ditto, deploying jetpack units
Global Defense Initiative Second Tiberium War Arsenal 
Mechanized walkers 