Windigo!

One thing that has always facinated me is the legend/lore of the Wendigo. The legend is based on the Alogonquin people's personal beliefs. It comes from the belief that if a human resorts to cannibalism, that they will become a monster. It is believed that humans can become Wendigo or posessed by Wendigo.
The Wendigo is described by Basil Johnston an Oijbwa teacher as the following (pulled from Wikipedia):

"The Wendigo was gaunt to the point of emaciation, its desiccated skin pulled tautly over its bones. With its bones pushing out against its skin, its complexion the ash gray of death, and its eyes pushed back deep into their sockets, the Wendigo looked like a gaunt skeleton recently disinterred from the grave. What lips it had were tattered and bloody [....] Unclean and suffering from suppurations of the flesh, the Wendigo gave off a strange and eerie odor of decay and decomposition, of death and corruption."

Wendigo are typically stories about indians, pioneers, or western folk that have to resort to cannibalism to survive. It's popularized by many horror stories. There has been an episode of Supernatural dedicated to them, as well a few movies recently. Namely, a movie called Wendigo and Ravenous to name a few. It always has to deal with the mythos of gaining a persons strength by consuming their flesh.

I always love the stories about Man changing into something else because of his own deeds. You never run out of material for the imagination. They are kind of like a mystical Frankenstein. A creature created with your own deeds, and your own need to survive...