Unagi

An Unagi is a species of eel that first appears in Super Mario 64. It is maroon-ish red-violet in color, also having yellow stripes on its back fin. The mega versions are proven in New Super Mario Bros. to be strong enough to destroy entire clusters of Brick Blocks, and they can also not be defeated with anything, even the Star power up.

Super Mario 64
In Super Mario 64 and its DS remake, one Unagi, known as Unagi the Eel, lives in Jolly Roger Bay and is the target mission of Can The Eel Come Out To Play? in which Mario must collect a star hanging on the eel's tail. Unagi can cause Mario to lose three health points if he comes in contact with him.

New Super Mario Bros.
After a long absence, they later appear in New Super Mario Bros. as a enemy that appears in underwater levels. Their name comes directly from the Japanese word for eels, unagi. During some 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 big version called the Mega Unagi.

Mario Kart Wii
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.

Mario Kart 7
They also reappear again in Mario Kart 7, making a cameo in the underwater zones of Daisy Cruiser and Koopa Cape.

Mario Kart Arcade GP DX
In Mario Kart Arcade GP DX, they make a small appearance in the underwater parts of the race track Tropical Coast.

Mario Kart 8
A single Mega Unagi appears in an underwater section of the Mario Kart 8 track Dolphin Shoals, where players race along its body for a short section and be able to perform Tricks.