Conversation so far:
what was the Pre-Crisis Comet?
Comet the Super-Horse was Supergirl's pet horse and, while in his human form as Bill Starr, her brief boyfriend
what comic books does Comet appear in?
Comet was introduced in the Superboy story in Adventure Comics #293 in February 1962.
Are there any other interesting aspects about this article on Comet?
Comet became a member of the Legion of Super-Pets, a group consisting of super-powered animals
what comics was Comet in?
Comet appeared regularly with Supergirl beginning in Action Comics #292 in September 1962.
who did Comet work with in the comics?
After meeting Supergirl, Comet went on a mission with her to the planet Zerox,
when was Comet created?
Comet was created in 1962.

Question: why did he change?
Self-contained rewrite: why did Comet change?