Maw-Ray

Maw-Rays, previously known as Unagi or simply Eels, are moray eels that first appear in Super Mario 64. They are maroon-ish red-violet in color with yellow stripes on its back fin. The mega versions are shown in New Super Mario Bros. to be strong enough to destroy entire clusters of Brick Blocks, and cannot be defeated with anything, even the Super 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). Maw-Ray is a pun on the moray eel and the word "maw".

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. The player character can lure him out of his sunken ship home in Jolly Roger Bay by swimming near and provoking him to attack. The player must then swim a short distance away and then swim back to the ship. Unagi will have left the ship, allowing the player 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 the player 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. If too much time passes, Unagi will return to the alcove, and have to be lured out again.

In later missions, Unagi circles the previous location of the sunken ship.

New Super Mario Bros.
After a long absence, Unagi appear in New Super Mario Bros. as an enemy in underwater levels. They only appear in World 4-3 and World 8-3. In World 8-3, Unagi swim around in a predictable pattern trying to hit Mario or Luigi. Some Unagi also live in small underwater coves, attacking anything in its vicinity. Unagi can be defeated with a Mega Mushroom and a Super Star. There is also a big version called the Mega Unagi.

Super Mario Odyssey
In Super Mario Odyssey, Unagi make an appearance in Seaside Kingdom under their current name, Maw-Ray. They are redesigned to have more detailed appearance with realistic anatomy as well as menacing, glowing eyes. They guard some of the kingdom's Power Moons, including a grotto where Mario must swim past several of them to get to the top of the Lighthouse. Similar to New Super Mario Bros., they attack by lunging out of their nesting spots at a high rate of speed before snapping their enormous jaws and receding, and can be positioned either horizontally or vertically. Before they emerge, a plume of sand can be seen coming from their caves, which appear to be blocked with sand. They reappear in the Mushroom Kingdom, where a few of them lunge upwards out of the clouds.

Mario no Bōken Land
The manga Mario no Bōken Land features adaptations of various games, one of them including Super Mario 64. The Super Mario 64 section of the book features Mario, Luigi and Donkey Kong traversing through several of the worlds in Super Mario 64; one of them, being an encounter with Unagi in Jolly Roger Bay.

Mario Party 4
In the minigame Manta Rings from Mario Party 4, a reddish-brown eel resembling an Unagi darts in front of the characters with a high-pitched noise as they pass a sunken ship, resembling the one from Jolly Roger Bay. If bumped into, three points are deducted.

Mario Kart Wii
Several Unagi 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. A closer one 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 one 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.