Sea nymph who was promised anything she wanted by Poseidon, if only she would spend a night with him. The nymph agreed, and after the night of love she asked to become a man. She wanted to be a invincible warrior, since she was tired of being subjugated by men.
As a man, the nymph's name was Caineos, and was soon elected king because of his many successes on the battlefield. He married and had a son, but soon the power wen to his head, and he wanted people to worship him as a god.
Zeus then ordered the Centaurs to kill Caineos, but since he was invincible, they could not kill him. Finally, they managed to drive him down into a hole in the ground with branches, and then dropped tree trunks on the hole so he was suffocated.
Cainis turned into a woman again after her death, and her soul flew away as a bird.