As he described to her telepathically, he was originally a centaur in ancient Greece named Biron. The witch Circe gave him a potion to turn him fully human after he prevented an evil sorcerer poisoning her water, but by mistake made him fully horse instead due to the Sorcerer. Unable to reverse the spell, she instead gave him superpowers, including immortality. The Sorcerer asked his teacher to help him against Biron and they were able to imprison him on an asteroid in the constellation of Sagittarius, which he had been born under. However, when Supergirl's rocket passed, it broke the force field, enabling him to escape. Later, after meeting Supergirl, he went on a mission with her to the planet Zerox, where a magic spell was cast that turned him into a human, but only while a comet passes through the solar system he is in. As a human, he adopted the identity of "Bronco" Bill Starr, a rodeo trick-rider, whom Supergirl fell in love with.