Hey, you know how wizards in DnD and other fantasy media have familiars? Little animal companions, that help them out and keep them company?
How about an option that was a huge axolotl that starts out cute and cuddly but then turns into this badass, amphibian mount that can fire pressurized jets of water? Magic can do fun things with living things and having it influence an initially harmless little amphibian is not that much of a stretch :P