Which third mythological animal should I represent my sector with?

So I'm writing a novel, a cross between Harry Potter and the Hunger Games, and I need help on figuring out the "mascot" for the third sector. These mascots are for a magic academy and will represent the sectors and which type of magic they use. I have Pheonix (Fire and air), Dryad (Earth and Light), and I need another mythological animal (preferably water based).

Can anyone please help?