The Púca (Old Irish for ghost), (also Pwwka, Pooka, Puka, Phouka, Púka, Pwca in Welsh, Bucca in Cornish, Pouque in Dgèrnésiais, Puca or Puck in English, Glashtyn, and Gruagach) is a creature of Celtic folklore, notably in Ireland, the West of Scotland, and Wales. It is one of the myriad of faery folk, and, like many faery folk, is both respected and feared by those who believe in it.
Morphology and physiology
According to legend, the púca 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. No matter what shape the púca takes, its fur is almost always dark. It most commonly takes the form of a sleek black horse with a flowing mane and luminescent golden eyes. 
If a human is enticed onto a púca's back, it has been known to give them a wild ride, though unlike a kelpie, which will take its rider and dive into the nearest stream or lake to drown and devour him/her, the púca will do its rider no real harm. The púca has the power of human speech, and has been known to give advice and lead people away from harm. Though the púca enjoys confusing and often terrifying humans, it is considered to be benevolent.
Certain agricultural traditions surround the púca. It is a creature associated with Samhain, a Pagan harvest festival, when the last of the crops is brought in. Anything remaining in the fields is considered "puka", or fairy-blasted, and hence inedible. In some locales, reapers leave a small share of the crop, the "púca's share", to placate the hungry creature. Nonetheless, November 1st is the púca's day, and the one day of the year when it can be expected to behave civilly.
In some regions, the púca is spoken of with considerably more respect than fear; if treated with due deference, it may actually be beneficial to those who encounter it. The púca is a creature of the mountains and hills, and in those regions there are stories of it appearing on November Day and providing prophecies and warnings to those who consult it.