A boggart page in Arthur Spiderwick's Field Guide to the Fantastic World Around You, from the Spiderwick Chronicles.
Boggart (also called a bogey, bogeyman, bogle or bugbear) is a term used for a creature in English folklore. It can be benevolent household guardian, malevolent trickster or mishcevious goblin-like creature. The name is derived from the Welsh "bwg".
Household boggarts can cause things to disappear, milk to sour, and dogs to go lame. Boggarts thought to live in marshes or swamps caused the disappearance of children.
Appearance
Boggarts vary in size and general appearance but many are depicted with humanoid features. Boggarts, though usually small, could be as big as a small calf. Some boggarts could take on the form of animals, such as horses.
The boggarts in Lancashire, England, were believed to have a leader known as Owd Hob, a horned, hooved creature similar in shape to a satyr.
Folklore
In some cases, boggarts were thought to be "buried", similar to vampires, with a stake driven through the body.
In one story, known sometimes as "The Farmer and the Devil", a farmer bought a patch of land that was inhabited by the boggart. When the farmhe attempted to cultivate the field the boggart got angry, but after much arguing they decided to work the land together and share the bounty. The farmer, however, being greedy, began to ponder a way to cheat the boggart out of his share. When they were debating what to plant, he asked the boggart, "Which half of the crop do you want for your share, the part below the ground or the part above it?" The boggart thought for a while before answering "The part below the ground". The farmer sowed the field with barley. At harvest time the farmer boasted a big pile of barley while all the boggart had to show for his work was stubble. It flew into a rage and screeched that next time it would take what lay above the ground.
The next time the farmer sowed the field with potatoes. At harvest time the farmer laughed as he claimed his massive pile of potatoes while the boggart was yet again left with nothing to show for his efforts. Simmering with rage, the boggart stormed off, never to return again.
A boggart as a full moon in Harry Potter and the Prisoner of Azkaban.
In Popular Culture
The boggart is commonly featured in fantasy related works, such as in the Spiderwick Chronicles by Tony DiTerlizzi and Holly Black, the Chronicles of Narnia (as boggles) by C.S. Lewis and in the Harry Potter series by J.K. Rowling, where boggarts change form depending on the greatest fear of the nearest person.