The Pooka

The Pooka, recte Puca, seems essentially an animal spirit. Some derive
his name from poc, a he-goat; and speculative persons consider him the
forefather of Shakespeare's "Puck". On solitary mountains and among old
ruins he lives, "grown monstrous with much solitude", and is of the race of
the nightmare. 

"In the MS. story, called 'Mac-na-Michomhairle', of uncertain
authorship," writes  Mr. Douglas Hyde, "we read that 'out of a certain hill
in Leinster, there used to emerge as far as his middle, a plump, sleek,
terrible steed, and speak in human voice to each person about
November-day, and he was accustomed to give intelligent and proper
answers to such as consulted him concerning all that would befall them until
the November of next year. 

And the people used to leave gifts and presents
at the hill until the coming of Patrick and the holy clergy.' This tradition
appears to be a cognate one with that of the Puca." Yes! unless it were
merely an augh-ishka [each-uisge], or Water-horse.

 For these, we are told,
were common once, and used to come out of the water to gallop on the
sands and in the fields, and people would often go between the mares
and bridle them, and they would make the finest of horses if only you could
keep them away from the sight of the water; but if once they saw a glimpse
of the water, they would plunge in with their rider, and tear him to pieces at
the bottom.

 It being a November spirit, however, tells in favour of the
Pooka, for November-day is sacred to the Pooka. It is hard to realise that
wild, staring phantom grown sleek and civil.

He has many shapes - is now a horse, now an ass, now a bull, a goat, now
an eagle.  Like all spirits, he is only half in the world of form.

Source: William Butler Yeats - Writings on Irish Folklore, Legend and Myth, published by Penguin Books 1993.