The wendigo is my candidate for the scariest monster in the entire world. Imagine an emaciated giant as white as snow, with the power to raise up blizzards and storms, and an insatiable desire for human flesh. That's one form of the wendigo, a traditional monster of the Algonquian peoples. The other form is arguably worse, because it's a non-corporeal spirit that can possess a person and turn him into a cannibal.
Wendigos (under various names) are part of the mythology of native peoples in the northeastern United States and parts of Canada. It was believed that if any man committed cannibalism- even to save his own life- he would inevitably become a wendigo, possessed by the need to kill and eat other people. Usually the first victims would be members of his own family.
The lumberjacks who worked in the Great North Woods included Frenchmen, Irishmen and many other nationalities, but they adopted the folklore of the wendigo into their own mythology. If a man suddenly started to behave strangely in the lumber camp- particularly if he claimed to be able to smell something that no one else could smell- he was thought to be possessed by the wendigo.
There is also a type of culturally-specific mental illness known as “wendigo psychosis,” which is the conviction or delusion that one is possessed by a wendigo and doomed to become a cannibal. The distinction between wendigo psychosis and actual wendigo possession seems to be a rather arbitrary one, predicated on the assumption that actual wendigo spirits simply cannot exist.