The wiki page on that one is really thorough, read it last month (depression) -
https://en.wikipedia.org/wiki/Patterson%E2%80%93Gimlin_filmPatterson was a schemer who self published a book about Bigfoot. He was trying to get funding to make a film about them, so he had a motivation, possibly thinking he'd get $ from a millionaire he met if he had proof. He told Gimlin ahead of time "not to shoot it" with a gun if they spotted one, which Gimlin, who recorded the footage, later said he regretted. But they both maintained to their deaths there was no hoax, the animal was real.
On one hand, "creature design expert" Stan Winston (famous for Alien and many other films) said it was a terribly cheap costume, but others disagree, including the costume designer for Planet of the Apes.
Years later, a costume maker came out claiming he made the suit, and a friend of Patterson claimed he wore it in the film, with their families backing up those claims somewhat.
Biology experts said that in conversing with Patterson, it was clear he didn't have expert level knowledge of bio-mechanics, and wouldn't have known some details they identify in the creature's walk or plaster footprints Patterson made. One university team was totally unable to recreate the walk using trained humans, including the gait and the distance covered.