"They are not creatures of your world. They are mutations." - Atlantean bigwig Atmir, on the Mogdaan and a couple of the film's other monsters.
Yeah, being a mutant alien fish thing from Mars (according to this film, Atlantis was founded by Martians), that is apparently one-of-a-kind, as Paul Victor's novelization would seem to suggest, definitely counts as a monster and not an animal in my book.