Unagi

The Unagis are a species of eel that first appeared en masse during the events of New Super Mario Bros. (although the first known Unagi was Super Mario 64 ' s Unagi the Eel). During underwater levels, Unagis would swim around in a predictable pattern trying to hit Mario or Luigi. Some Unagis also live in small underwater coves, attacking anything in its vicinity. There is also a subspecies called the Mega Unagi.

Several Unagis reappear in Mario Kart Wii, where they can be seen while traveling through the underwater tunnel in Koopa Cape, poking their heads out of various Pipes. Another, perhaps a Mega Unagi, can be seen swimming outside of Chain Chomp Wheel.

Trivia

 * "Unagi" is the Japanese word for "eel."