The Boston Dynamics Big Dog is only the latest in a long line of semi-autonomous cargo carriers developed for the US military. Back in the late 1960's, GE unveiled the Big Dog's spiritual predecessor: a mammoth mechanical pack mule strong enough to push Jeeps around like Matchbox cars.
Developed in 1965 by GE engineer Ralph Mosher and dubbed the "Cybernetic Anthropomorphous Machine" (or CAM), this 3,000lb experimental behemoth was designed for the same purpose as the Big Dog: to carry heavy equipment for infantry forces over rough terrain.
As you can see in the video above, the one-story tall CAM wasn't particularly nimble on its feet, required an on-board operator, and only had a top walking speed of five mph. However it made up for its pokiness with feats of brute strength—like the ability to free a bogged-down Jeep from muddy mires and demolish battlefield obstacles with a single kick of its front legs.
The Walking Truck was a promising development platform but soon proved too unwieldy to control for extended periods and too slow for deployment in the field. GE eventually ended the CAM program, though you can still catch a glimpse of the prototype itself at the US Army Transportation Museum in Fort Eustis, VA. [GE - Wiki]