Shapeshifting

Stories of 'shapeshifters' likely originate from the practice of astral projection and it's many analogues.
When exploring the many and varied astral realms it can be useful to employ an astral body suitably 'tuned' to the ambience of the place. A bestial form may be better suited to the lower, more emotional end of the astral spectrum, with more angelic forms coming into play in the higher strata.

Since most shamanic practices I've heard of involved some form of astral projection, it would seem that this would be the source of the stories you've heard. As these techniques filtered down into ears unfamiliar with the context, these became tales of shapeshifters, werewolves, ghouls, etc., who could change their physical body -- either at will or involuntarily. Shapeshifting is plausible, but perhaps not neccesarily by the definitions of which we are accustomed.If I decide I want to "become" a certain thing, usually an animal form, there is nothing stopping me from actually doing this.  I am an eagle, I am a crocodile. I am human. I am bigfoot.  Do you see?