An imaginary mythical creature who lives under the bed and in many cultures it is believed by little children that if you put your foot out of the bed blanket/comforter during night time, it may come out and eat your foot, (may be used to frighten children into good behavior).
Someone who is a foo (fool) and a beast (someone who's nether regions surpass that of a bears, and it resembles more of a mythical hairy beast). See also Foobear. It can also be pronounced foobeas' in ebonics.