Mythical Creatures....?

This isn't exactly my field but with the area i live in this is what came to mind. Skin walkers. This is basically a Native American, mostly Navajo, belief that certain people when an evil enough act is committed can gain the powers transform themselves into the animals that they require the abilities of. The Navajos that i know culturally believe that the skin walker can be a "mutated" form of the animal. This sounds very much like what your friends saw. They are also described as "fast, agile, and impossible to catch" which also describes your creature. Wikipedia has a pretty good definition, but know a navajo to get some wicked stories.