Pooka

Pooka : according to Irish tradition and folklore this creature is a mythical supernatural creature which assumes the guise of a malevolent horse.
Useful Links
Recommended Reading ...

Irish Wonders: The Ghosts, Giants, Pookas, Demons, Leprechawns, Banshees, Fairies, Witches, Widows, and Other Marvels of the Emerald Isle.
Spirits, Fairies, Gnomes, and Goblins: An Encyclopedia of the Little People.

is universal because they are known by various folkloric names including brownie (English and Scottish folklore), elf (German folklore), dwarf (Teutonic and Germanic folklore), troll (Norse folklore), gnome (Europe, popularized by Paracelsus), pooka ...

See also: Spirit, Fairies, Demon, Signs, Spell