Red Coin Fish
From the Super Mario Wiki
The Red Coin Fish is a fish whose body is made out of many Yellow Coins and eight Red Coins. Mario must propel himself through the water with F.L.U.D.D. to collect the eight Red Coins from the body of the fish during the events of "The Red Coin Fish" in Noki Bay – Super Mario Sunshine's eighth level. This is the only level in Noki Bay in which the Red Coin Fish makes an appearance. Even though its body is composed of coins (which are inanimate objects), the fish still appears to be alive. Usually, the Red Coin Fish will just swim around in the water, but occasionally, it will completely separate into the multiple individual coins that comprise it, which will then float around randomly in the water. It will then reform back into a fish shape after a few seconds. When Mario successfully collects all eight Red Coins from the body of the fish without running out of air, a Shine Sprite will appear at the bottom of the bay.