Unagi the Eel
From the Super Mario Wiki
Unagi the Eel is a large Unagi found in Super Mario 64 and its remake Super Mario 64 DS. Mario or his friends can lure him out of his sunken ship home in Jolly Roger Bay by swimming near and provoking him to attack. After about three attempted bites, he will leave the ship, allowing Mario and his friends to enter it via the ship's window. Once inside if Mario opens the chests in the correct order, the ship will surface, causing the water inside the ship to drain. Mario can now get the Power Star.
Unagi the Eel is seen again later, this time making a water cove his home. Again Mario and friends can lure him out in the same fashion as before. The water cove is not accessible but there is a Power Star attached to the eel's tail. To gain it Mario and friends must simply touch it.
In later missions, Unagi the Eel circles the previous location of the sunken ship.