From the Super Mario Wiki
Eely-Mouth is an eel only appearing in Super Mario Sunshine. It lives in the Nokis' ancestral home, a massive underwater chamber below Noki Bay, and its gingivitis was the cause of the widespread pollution in the area. In order to purify Noki Bay's water, Mario must clean Eely-Mouth's eight teeth, three of which are rotten and fall out after being sprayed with F.L.U.D.D.'s water. One of its teeth is revealed to be golden once it is cleaned off. Eely-Mouth has two separate pairs of eyes on either side of its mouth; one pair appears to be male, and the other female (though the character is referred to as a "he" by the Noki Elder).
At the start of the episode, The Noki Elder will explain to Mario that he has finally figured out the problem as to why the bay is polluted, and will send him to the ocean floor to fight the giant eel. Due to its intense suffering, Eely-Mouth is aggressive and will spin around very fast, and then rise upward and open its mouth. This is when Mario can attempt to clean his teeth, by spraying them with F.L.U.D.D. After some time, Eely will suck in water, and Mario as well, if he is too close. If this happens, Mario is quickly spat back out and will lose air. Large toxic purple bubbles regularly come out of Eely's mouth, and will damage Mario if touched. If Mario sprays the bubbles with water from F.L.U.D.D., they will pop into a cluster of smaller regular bubbles that do not harm Mario, but replenish his air supply. Once all the teeth have been cleaned, Eely-Mouth retreats back into its cave, and spits out its golden tooth, which turns into a Shine Sprite which falls to the bottom, that Mario must collect. As the eel recedes into its cave, it also leaves behind twenty Coins in a heart-shaped formation, which float just above the mouth of its cave (collecting them or not is optional), most likely as a sign of thanks. After Eely-Mouth has completely disappeared, F.L.U.D.D. comments, "Remember to take proper care of your teeth!"
In Mario Golf: Toadstool Tour and Mario Golf: Advance Tour, Eely-Mouth's name appears on the scorecard.
Names in other languages
||From french word "Anguille" and french name "Guy"
||Portmanteau of anguilla (eel) and Lillo (uncommon Italian male name)