This is CAM (cybernetic anthropomorphous machine), a "walking truck" designed by Ralph Mosher at General Electric in 1965. It may not be as rough-and-tumble as Boston Dynamics' BigDog but it was certainly more fun because the operator rode inside of it! From Wikipedia:
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The stepping of the robot was controlled by a human operator through foot and hand movements coupled to hydraulic valves. The complex movements of the legs and body pose were done entirely through hydraulics. The hydraulic fluid and pressure was supplied through an off-board system. The walking truck was one of the first technological hardware design applications to incorporate force feed-back to give the operator a feel of what was happening.