Here's a Bigfoot theory I haven't heard before. Apparently there are some in the Mormon church who hypothesize that Bigfoot may actually be Cain, condemned to walk the earth forever. Matt Bowman provides some scholarly elaboration on this theory on the Mormon Mentality blog.
Apparently the Bigfoot-Cain connection traces back to a story told by an early leader of the Mormon church, David W. Patten. Patten claimed that in 1835 he encountered Cain walking along the side of the road. He wrote: "He walked along beside me for about two miles. His head was about even with my shoulders as I sat in my saddle. He wore no clothing, but was covered with hair. His skin was very dark."
Hmm. That sounds kind of like Bigfoot. At least, that's what some Mormons have apparently concluded in recent decades. Bowman writes: "Cain’s identification as Bigfoot has provided Mormons with a way to assimilate the claims of folktale with new conceptions of what Cain, the embodiment of evil, should be like."
So if Bigfoot is Cain, maybe Nessie is really the snake from the Garden of Eden.