Unagi

Unagi is a species of eel that first appears in Super Mario 64. It is maroon-ish red-violet in color with 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 cannot be defeated with anything, even the Star power up.

Unagi (うなぎ) is the Japanese word for freshwater eel, especially the Japanese eel, Anguilla japonica. Note that saltwater eels should be called anago (穴子).

Super Mario 64 / Super Mario 64 DS
One Unagi, sometimes referred to as Unagi the eel, appears 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, thus allowing Mario to get the Power Star.

Unagi is seen again later, this time making a water cove his home. Once 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 circles the previous location of the sunken ship.

New Super Mario Bros.
After a long absence, they appear in New Super Mario Bros. as a enemy in underwater levels. They only appear in World 4-3 and World 8-3. In World 8-3, Unagis 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. Unagis can be defeated with a Mega Mushroom and a Super Star. 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 / Mario Kart 8 Deluxe
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 can perform Tricks from its undulating body. It returns in the enhanced Nintendo Switch port Mario Kart 8 Deluxe.

Super Mario Odyssey
In super Mario Odyssey, they are found in the Seaside Kingdom.