I've seen Kathy Strain being mentioned here, so we have something to start out on about the elusive bigfoot cave paintings. I've found this exquisitely referenced post that practically destroys her arguments about the Yokuts bigfoot. Here's the gist: She claims that the Yokuts people have a cave painting depicting a family of bigfoots and that they have a creation myth about these creatures. There are some issues with these claims. Firstly, there are no other bigfoot creation stories anywhere else in the West. Secondly, there are no other bigfoot cave paintings anywhere. The only source for the bigfoot creation stories are from Kathy Strain, there is no record of them anywhere outside her writings and bigfoot websites, even though the Yokuts legends are really well-documented. Furthermore, all the other scientific material written about the paintings suggests that they are of anything but bigfoot. Instead, they are evil spirits/bears/mountain lions/people. The only academic who seems to have reached the bigfoot conclusion is Kathy Strain. The Yokuts do have a "Hairy Man" in their myths, but just because something is hairy and walks on two feet that does not equate it with bigfoot, just look at Robin Williams.
So, there we have it. The cave paintings are not clear, but they are most probably not bigfoot.
I agree that you can not use them as proof, and I was by no means saying you should accept something simply for the fact it appears in folklore. But you should also not discount something just due to folklore and I was not just referring to the alleged Bigfoot example, just in general. I guess I somewhat misunderstood what you said, and I thought you were claiming folklore is just simply that with no basis in reality, so I do apologize for that, though I still stand by my views on the topic.