The only problem with the most obvious answer which would be "It's a man in a suit" is that there just wasn't anybody good enough back then to do that. Not even the best who was John Chambers. American Werewolf in London's Rick Baker might have been able to duplicate the patty footage at the absolute peak of his hair suit work which would have been Tarzan - The Legend of Greystoke in around 1984 .. I'd turn the music down for this but it demonstrates who good Baker was in that year .. all of these apes are fake costumed actors or puppets. ..
The first time I learned about body/mass muscle structure being faked for a film was from around the same time with Marc Shostrum's crew including the now more famous than him KNB F/X working for him. They sewed bags of ball bearings under a foam latex body suit that was about the same size as how we view patty in that late 60's footage. They did that do create an obesity look with the bottom and breasts of the character wobbling for Evil Dead 2. Tiny details.
Facially, make up artists were able to create something as convincing as patty's face since the 1930's.
'Tying' all of that together though, on location with a cheap camera in a remote forest in '69 with a presumably a low budget would have been extremely difficult to practically achieve. If it is a man in a suit, I'd argue it's more likely to be a woman in a suit if it was faked. Because of the more nimble way it moves over the rough terrain. It would also be more likely to not be a single suit but compartments of a suit as well as a separate facial make up. Or it's the real deal. If it's fake, it was very advanced for 1969.