If you happen to spot someone in a field on all fours, chewing grass, and exhibiting other cow-like behaviors such as mooing, they may be suffering from Boanthropy. Boanthropy is a rare psychological disorder that causes a human being to believe that they are a cow. Those inflicted with Boanthropy behave accordingly and attempt to live like cows. I don't think I've met anyone with Boanthropy before, but perhaps I will someday.
The most famous sufferer of this disorder was Nebuchadnezzar II, who in the Book of Daniel "was driven from men, and did eat grass as oxen". Carl Jung would subsequently instance 'Nebuchadnezzar…[as] a complete regressive degeneration of a man who has overreached himself'. According to Persian traditions, the Buyid prince Majd al-Dawla was suffering from an illusion that he is a cow, making the sound of a cow and asking that to be killed so that his flesh could be consumed. He was cured by Avicenna. " Boanthropy still occurs today, and "It has been suggested that hypnotism, suggestion and auto-suggestion may contribute to such beliefs.