The pooka (púka, phooka) comes from Irish mythology.  He is generally considered a mischievous, and sometimes malevolent, shapeshifting spirit.  Stories vary – in some he merely tramples crops, but in others he has been known to lure unsuspecting men and children to ride to their deaths.  He commonly takes the form of a wild black horse with sulfur yellow eyes, a large black eagle, or a goat, among other things.

In Folk and Fairy Tales of Ireland, W.B. Yeats writes: “The Pooka, recté Púca, seems essentially an animal spirit…On solitary mountains and among old ruins he lives, 'grown monstrous with much solitude', and is of the race of nightmare…Like all spirits, he is only half in the world of form.”

More Info:

“Púka” in Wikipedia

“The Pooka” from A Field Guide to Irish Fairies

Recommended Read with a pooka character:

War For The Oaks by Emma Bull