Basic Information
The Bugbear is a creature of Anglo-Celtic legend, etymologically a goblin or spirit bear1, often fulfilling the same narrative role as the bogeyman - an abductor and minatory device for children. Modern, fRPG adaptations have tended to transform the bugbear into a large, hairy humanoid thing - less supernatural and more aggressive, but often retaining a talent for stealth and abduction.
So far, the most egregious adaptation has been an art error in the first editions of the World of Greyhawk campaign setting that presented the bugbear as a bear with a pumpkin lantern for a head. This was purged in later editions, but other fRPGs (such as Runequest) picked the idea up and ran with it as a separate creature.