Unagi

Unagi is a giant eel that lives in a cave at Jolly Roger Bay, featured in the game Super Mario 64. Unagi comes out of his cave everyonce in a while, and when he does come out, you'll see a star at the end of his tail.

He also is in the game New Super Mario Bros. for the DS. In one of the levels, he will swim across the whole screen, destroying everything thats in his path. All while this happens, mini Eels resembling Unagi will swim across the screen as well, and pop out of mini caves.