From Tamim Ansary's Scariest Monsters list:
The windigo is a Native American nightmare, invented in the subarctic by the Cree and Ojibwa peoples.
Let's set the scene: The subarctic is about 5 million sq km (2 million sq mi) of forest and tundra inhabited by fewer than 60,000 people. We're talking lonely. Winters last forever up there, and winter nights are long, cold, and dark.
In times past, the people who lived in this region spent winters holed up in their shelters, rationing out their dwindling food supplies, and telling tales. The most dreadful of these stories featured the windigo, a friendless creature that lives alone in the forest. It's 6 to 9 m (20 to 30 ft) tall and has a lipless mouth and jagged teeth. Its footprints in the snow are full of blood and you can hear its hissing breath for miles.
It eats people. And that's the good news.
The bad news is, if a windigo catches you alone in the forest, it can possess you. Then you turn into a windigo yourself--a mindless cannibal.
I think I see the underlying fear that glimmers through this story. Survival in the subarctic was a desperate struggle. Often, food ran out before winter did. Sometimes, well.... Cannibalism happens.
There you have the real horror of the windigo legend. Ask not what some monster might do to you; ask what you might do to someone should you become a monster.