The Pooka is a creature of Celtic folklore.
It is one of the myriad fairy folk, and, like many fairy folk, is both respected and feared by those who believe in it.
According to legend, the Pooka is a deft shape shifter, capable of assuming a variety of terrifying or pleasing forms, and may appear as a horse, rabbit, goat, goblin, or dog.
You might say Pookas do not exist and that is probably right...
but the idea of an invisible friend, like a 6-foot rabbit called Harvey, is not so bad!