Definition of "pooka"

This is certain; Moggy was by no means so great a fool as Betty in respect of hobgoblins, witches, banshees, pookas, and the world of spirits in general. She eat heartily, and slept soundly, and as yet had never seen the devil.

1863, Sheridan Le Fanu, The House by the Churchyard

The Pooka is an animal of which, partly from the darkness of the night, and partly from the darkness that he carries about with him, the precise outline, especially of the head and neck, can never be distinguished.

1833 July, “Laurie on Grand Juries”, in The Westminster Review - Volume 19, page 88

P-O-O-K-A. Pooka. From old Celtic mythology, a fairy spirit in animal form, always very large. The pooka appears here and there, now and then, to this one and that one. A benign but mischievous creature. Very fond of rumpots, crackpots, and how are you, Mr. Wilson?

1944, Mary Chase, Harvey (play)