...Something unique that sets your game apart from others. For example, Final Fantasy had chocobos -- those are as unique as can be. Morrowind has silt striders -- who'd've thunk of those? Secret of Mana has a giant furry dragon... and so on.

If you're stuck for a unique idea, how about some kind of giant bird -- a cross between an eagle and a goose/duck -- that fills the role of an aerial riding beast? It has talons for swooping attacks, of course, and it must be able to land on the water. (Or, if you really want to be unique, only be able to land on the water. You could justify that by saying that the legs aren't strong enough to hold the bird's entire weight, which isn't exactly unbelievable, considering it is a big bird (this is assuming that even with hollow bones, it is still too heavy to hold itself up).)