Nightmare
From MTG Salvation Wiki
The original Nightmare is a powerful, horse-like spirit that draws its power from the swamps. See also Caliphear the Nightmare. Creatures from dementia space are also called nightmares.
[edit] Dementia creatures
Dementia casters are able to summon twisted forms of creatures they once killed. Mechanically, these nightmares have triggered abilities that remove something when they come into play and return it when they leave play.
