Puck, also known as Robin Goodfellow, is a domestic and nature sprite, demon, or fairy. He is one of the most popular beings in English folklore of the last thousand years has been the faerie, goblin, devil or imp known by the name of Puck or Robin Goodfellow. The Welsh called him Pwca, which is pronounced the same as his Irish incarnation Phouka, Pooka or Puca. These are far from his only names.
As a shape-hifter, Puck has had many appearances over the years. He's been in the form of animals, like how the Phouka can become a horse, eagle or donkey. He is also depicted a rough, hairy creature in many versions. One Irish story has him as an old man. He is been pictured like a brownie or a hobbit. In a 1785 painting by William Blake, he looks like Pan from Greek mythology. In a 1841 painting by Richard Dadd, Puck looks like an innocent child. And a modern cartoon show portrays him as a silver-haired elf.
Puck used his shape-shifting to make mischief. For example, the Phouka would turn into a horse and lead people on a wild ride, sometimes dumping them in water. The Welsh Pwca would lead travels with a lantern and then blow it out when they were at the edge of a cliff. Being misled by a Puck (sometimes the legends speak of Pucks, Pookas and Robin Goodfellows in the plural) was known in the Midlands as being "pouk-ledden."
Puck is one of the faeries known as hobgoblins or just hobs. Hob is a short form for the name Robin or Robert ("the goblin named Robin".) Robin itself was a medieval nickname for the devil. Puck was not only famous for shape-shifting and misleading travellers, he was also a helpful domestic sprite much like the brownies. He would clean houses and such in exchange for some cream or milk. If offered new clothes, he'd stop cleaning. There are stories of the Phouka and Pwca doing similar deeds.