CAENIS started off as a beautiful mortal maiden who was date-raped by POSEIDON. She was so traumatized that, for once in his life, POSEIDON felt guilty. He offered to make amends in any way he could. "I never want to go through that again," she said. "Turn me into a man."
Her wish was granted, the sex-change was performed, and she became an invulnerable warrior. Of course she — sorry, he — could not go around with a lady’s name. Everyone would talk. So he changed it to Caeneus and went off boar-hunting with the Argonauts.
Many battles later, Caeneus came up against some centaurs who, unable to kill him, buried him alive under a mountainous pile of rocks and tree trunks. But he simply flew off, having turned at last into a bird. Allegedly a female one.