Boanthropy is a psychological disorder in which a human believes himself or herself to be a bovine, that is a cow or a bull. The most famous case of boanthropy occurred in King Nebuchadenazzar, the Babylonian King, who in the Book of Daniel was "driven from men and did eat grass as oxen".
The disorder usually begins by becoming vegetarian. It begins as a sort of dream in which the person after mooing uncontrollably and acquiring a strange taste of grass, wakes up in a field somewhere, unable to do anything. They are obliged to crawl on all fours, eat only grass and green vegetables, but prefer not to use their hands.
The exact cause of boanthropy is unknown but a manifestation of anxiety is believed to be a key component. It is also possible that this state is related to hypnosis in susceptible individuals.