Under various state animal cruelty statutes, a "companion animal" is an animal that is commonly considered a pet. Under New York law, the crime of aggravated cruelty to animals applies only to abuse of domesticated animals. In the landmark case of People v. Garcia, it was finally settled that pet goldfish do indeed qualify as "companion animals" -- just in case anyone was considering taking away their little plastic castle.
In a cosmic coincidence, the very funny Supreme Court Jester has taken up the goldfish/companion animal theme. He has written a detailed post about the rights of goldfish -- including goldfish divorce law!