Originally, Caeneus was a beautiful maiden named Caenis. Poseidon raped her, and afterwards promised to grant her anything she wished; she wished to become a man, so that nothing like this could ever happen to her again. The god granted her wish, and in addition, made her/him invulnerable to all weapons.
At the wedding of Pirithous, when fighting broke out between the Lapiths and the Centaurs, Caeneus slew many of the Centaurs but remained unharmed himself. Finally a mob of Centaurs began piling pine trees upon him, because they could not kill him, and he was crushed or smothered beneath the great weight. After his death, he flew away as a bird.