Due to the events depicted in the 1985 limited series, Crisis on Infinite Earths the first character's stories are no longer considered to be canon within DC's main shared universe, known as the DC Universe.
One of a series of super-powered animals, including Streaky the Supercat and Beppo the Supermonkey, that was popular in DC's comics of the 1960s. Comet was Supergirl's pet horse and while in his human form as Bill Starr, her brief boyfriend. Comet also had a brief romance with Lois Lane in her comic book.
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, but by mistake made him fully horse instead. Unable to reverse the spell, she instead gave him superpowers, including immortality. Later, after meeting Supergirl, he went on a mission with her to another planet, 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.
Comet made sporadic appearances in comic stories through the 1960s, and even became a member of the Legion of Super-Pets, a group consisting of the superpowered animals mentioned above.
Soon, it was learned that Comet's male form was originally Andrew Jones, a (male) jockey, who had been trampled by horses and "rebuilt" by an organization called "The Stable" as a superhuman with equine DNA. He rebelled against the organization and began operating as a superhero. On one of his first missions he attempted to save a despairing Andrea Martinez (who had just come out to her parents, and been rejected) from an avalanche, but they both died. As with Matrix and Linda Danvers (who had merged into Supergirl, the Angel of Fire), this caused them to combine into one being: the Earth Angel of Love. Comet originally was in love with Supergirl, and since he was the Angel of Love, made her have feelings for him too, but she rejected him when she learned he was also a woman. This opened the door for the third Angel, Blithe, the Angel of Light, to use her powers to exploit Comet's heartbreak, and turn him against Supergirl. She enabled Comet to fully embrace his angel powers, transforming himself into a winged centaur.
Blithe deceived Comet into joining the Carnivore (a powerful demonic creature who despised the Earth Angels, and sought their power). The Carnivore lost control, however, when Andrea abandoned her anger, learning that her mother had died, and had left a video apology for her actions. The three Earth Angels worked together to stop the Carnivore. Comet began a relationship with Blithe, since she loved both his forms (hinting that Blithe is bisexual).