Comet is the name of two fictional comic book characters owned by DC Comics whose adventures have been published by that same company. The first character was a sapient horse with magical powers who was once a centaur in ancient Greece. The second character is a shapeshifter with three forms (male, female, and winged centaur). Both characters are connected to the Superman family of titles.

Comet the Super-Horse is a fictional character that appears in comic books published by DC Comics. Comet was introduced in the Superboy story in Adventure Comics #293 in February 1962, then appeared regularly with Supergirl beginning in Action Comics #292 in September 1962.  Comet was one of a series of super-powered animals, including Streaky the Supercat and Beppo the Supermonkey, that were 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 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.  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 aforementioned super-powered animals.  A traditional equine Comet is partially seen in Legion of Three Worlds #1. He is part of a display in the museum Superboy-Prime visits. The museum does have displays of the supermen of the multiverse and the Kristen Wells Superwoman so it is unclear whether this means Comet has returned to regular continuity. Answer this question using a quote from the following article:

how did he change?
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