A female to male mythological figure viewed as a 'scorner and rival of the gods', Kainis (or Caenis) was the daughter of Elatos of Thessaly and Hippia, one of the Lapithae of Gyrton. Poseidon, in love with her, but refused, first raped Kainis, but later Poseidon offers her a favor. Then she asks to become a man, and therefore invulnerable to rape. Poseidon transformed her into the invulnerable fighter Kaineus (Caeneus).
However, Kaineus committed a sacrilege towards Zeus, and took part in the Argonautic expedition and the Calydonian boar-hunt. At the marriage of Pirithoüs, the Centaurs, who had to punish him with death for his sacrilege, found him invulnerable. They "could neither bend nor slay him; but unconquered and unflinching he passed beneath the earth, overwhelmed by the down rush of massy pines" (Apollonius 2).
Kaineus was such an extraordinary fighter that Nestor remembered him, and mentioned him, in his speech to the young soldiers of the Trojan War in an effort to make them realize how insignificant they were. (Homer Iliad 20).
Ovid describes a later metamorphosis of Kaineus into a bird (Ovid Met. 12.189-209). Poseidon had created an undefeatable warrior that was remembered for generations, but even he could not stand up to the wrath of Zeus.