BigDog was a dynamically stable quadruped military Robot created by Boston Dynamics. Instead of wheels, BigDog used four legs for movement, allowing it to move across surfaces that would defeat wheels.
It was funded by the DARPA in the hopes that it would be able to serve as a robotic pack mule to accompany soldiers in terrain too rough for vehicles, but the project was shelved after BigDog was deemed too loud to be used in combat.