Runauturungo
The runauturungo is a puma-man from the traditions of Bolivia and northwestern Argentina who attacks travelers, robbing them of everything they carry. It is said to be the fruit of a pact with the devil, and its desire is to take revenge on the humans who wronged it.
All the runauturungo needs to do is roll about on a puma skin that it always keeps in its belongings, and then, after uttering an incantation, it transforms into the animal itself.