According to Cryptozoologists Loren Coleman and Patrick Huyghe, Lakes Bear, Utah and Powell (all in Utah) and some other places in North America are haunted by giant beavers. Only one sighting from man-made Lake Powell is explicitly of a giant beaver, the rest are vague, remarkably un-beaver-like (“alligator-like jaws”, “reptilian”, etc.), or legends. Even among cryptozoologists, this doesn’t seem to be a very popular cryptid.
There were real giant beavers (Castoroides), which weren’t towering sloth-like beasts.
Coleman, L. & Huyghe, P. (2003) The Field Guide to Lake Monsters, Sea Serpents, and Other Mystery Denizens of the Deep.
I read lots of bizarre hypotheses by cryptozoologists, so much so, I’ve started another blog. The (Internet) world needs to know about the Giant Beaver Hypothesis.