It's a guy in a suit. The original was purposefully shaky and Patterson most likely played with the film speed to make it look less human. There's another gif that focuses on the feet, which look suspiciously like old fishing waders with padding on the bottom. It's all in book by Greg Long called The Making Of Bigfoot. Here's another one showing Bob Heironimous, the guy who claims he was in the suit in 1967:

Sorry bout the derailment. Right now Mallocup's avatar is making laugh every time I look at it.