Tantabus

The Tantabus is a creature from the My Little Pony: Friendship is Magic universe.

The Tantabus is a dream parasite that infects one's dreams and turns them into nightmares. The more negative energy it feeds on, the more powerful it becomes. If it becomes too powerful, it'll escape into the physical world and turn it into a living nightmare.

It was created by Princess Luna as a form of self punishment for her actions as Nightmare Moon.