I want to suggest a creature!
Species Name: Giant Koi
Diet: Seaweed, smaller fish, plants etc.
Habitat: Lakes, oceans, and bays
Appearance: Big and brightly colored, with various patterns and spots
Description: Well, the humans sometimes may not be able to take dragons out onto sea, or if they don't have a water-like dragon, they ride the backs of these giant koi! There was a legend that they hold mana in their bellies, but that legend has not been proved.
My Inspiration, the Elephant Koi featured in Avatar the Last Airbender
I even made a baby koi for my signature!