A werewolf is an animal from folklore which can change from human to wolf and back again and is believed to consume human flesh or blood. (Wer is an Old English term for man.) While there are no documented cases of any human turning into a wolf and back, there are documented cases of humans who believed they were werewolves. To suffer from such a delusion is known as lycanthropy.
Some have speculated that certain excessively hairy individuals resemble wolves and that the legend of the werewolf may have a basis in the genetic disorder known as hypertrichosis or in some other endocrine disorder, such as adrenal virilism, basophilic adenoma of the pituitary, masculinizing ovarian tumors, or Stein-Leventhal syndrome.