Conversation so far:
What is Comet?
Comet is the name of two fictional comic book characters owned by DC Comics whose adventures have been published by that same company.
Who's pet was Comet?
Comet was Supergirl's pet horse and, while in his human form as Bill Starr, her brief boyfriend.
Who were some other super animals like Comet?
The Legion of Super-Pets is a fictional team of super-powered pets in the pre-Crisis DC Universe. Members include Krypto the Super-Dog, Streaky the Supercat, and Comet the Horse.

Question: Where did Comet first appear?
Self-contained rewrite: What comic book issue did Comet first appear?