Wendigo
The Wendigo probably existed, just not as a creature. People in the far north who survived a brutal winter by eating a family member had a psychological escape hatch from the guilt and horror by convincing themselves they were transforming into a ravenous, murderous beast. They would continue to do it in a hysteric delusion that they had no control over it. Wendigo hunters would then have to put them out of their misery and perform a shamanic ritual to assure the rest of the tribe that the taint wouldn’t spread.
It’s actually an incredibly fascinating study into culturally specific mental illness. The lengths the mind will go to in order to avoid dealing with a traumatic event are so extraordinary that in that culture, they would actually continue to murder and cannibalize fellow tribe members under the delusion they had transformed into a monster.