From the Super Mario Wiki
A Lunge Fish is a big fish that will eat Yoshi and Baby Mario whole in Super Mario World 2: Yoshi's Island, Yoshi's Island: Super Mario Advance 3, Yoshi's Island DS, and Yoshi's New Island. Its name is a pun on the type of fish known as the "lung fish".
Lunge Fish hide in the water, and, like their name implies, will lunge at Yoshi as he passes by. Occasionally, a Lunge Fish will stick its head out of the water. Yoshi can then throw an egg at the fish to momentarily stun the creature. In one level of Yoshi's Island DS, Yoshi must travel in a boat over a lake infested with Lunge Fish. If Yoshi falls in the water, a Lunge Fish will attempt to swallow Yoshi and the baby he is carrying whole similar to how a Boss Bass does.
The Lunge Fish also appears as a playable character and an opponent in Tetris Attack, being paired with Flopsy Fish. In this game, it has become a slight friend of Yoshi's, enchanted by Bowser and Kamek to work for them. It also notes that it really doesn't like Yoshi much, since Yoshi gives it indigestion. It replaces Neris, the sea fairy from the Japanese version of the game.
In volume 14 of the Super Mario-Kun a Lunge Fish is much bigger than Yoshi and is able to use electricity to electrocute its enemies. Mario sees a Winged Cloud that gives out a Super Blue Watermelon. Mario makes Yoshi eat it, which then freezes the Lunge Fish.
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