A Set of glowing teeth that randomly spawns around the map and shifts position every once in a while. If you see it glowing on the wall, don't try to walk past it or it'll devour you and it'll either spit you out at a random spot in the map, a secret area, or end the run (whatever you feel like would be best for the game design). You cannot sprint by it, but you can wait it out. The flashlight has no effect on it.
The point of this Nightmare would be to block players who have a specific path memorized to win the game. Sort of throwing a random road block in the way causing people to either stop and wait, risking death via another Nightmare, or finding an alternate path. I hope you like this idea! I'm sure I can brainstorm many more! I love making random monsters with random mechanics!