Pwwca (pronounced roughly as "Phooka" or "Pooka") is a Celtic spirit or shapechanging faerie. It was best known for taking the form of a horse and then bring its rider to death by running over a cliff (and then changing into a bird) or into water (which makes it similar to the kelpie). It's from this word that Shakespear came up with the name "Puck" from "A Midsummer Night's Dream." (It's also from this name that I got my dog's name. What better name for a dog than a creature that lures travellers to their deaths.) It's also known as being a 6' tall rabbit from the movie, play, and book "Harvey."
There were other spirits called bwca. (I've also seen bwwca, so I'm not sure which is more correct.) The bwca were less mischievious, more helpful, so it is possible that they were an entirely different creature. These spirits later became known as "brownies" and were the basis of the childrens story "The Elves and the Shoemaker." But, the similarity in names is most likely not a coincidence. Many stories had these mischievious pooka suddenly playing similar roles to the otherwise helpful bwca.
Some say the foundations for the legends of the pooka and bwca came from a central European god named "Boga," which was a nature god (similar to the god Pan). From his name we also get Bog and boogie man. The Slavic word "Bog" actually meant "Almighty," and I've seen some places say that's where we derived the English word "God" in the first place.
The Ooolong Man tells me that Pooka/Pwwca is also a where we derive the word "pixie."