Nep-Enut

Nep-Enut, also known as Submarine Nep-Enut or Giant Nep-Enut , is a water-based, blue variant of Gargantua Blargg that hides underwater in the games Super Mario World 2: Yoshi's Island, Yoshi's Island: Super Mario Advance 3, Yoshi's Island DS, Yoshi's New Island, and Yoshi's Woolly World. Nep-Enut will remain submerged until Yoshi approaches it. Then, the monster will rise, blocking Yoshi's path. Yoshi must shoot inside Nep-Enut's mouth with an egg for it to submerge in the water (and only its eyebrows will remain visible). If Yoshi jumps over the giant monster, Nep-Enut will give chase. Like Gargantua Blarggs, Nep-Enuts are increasingly bigger and more detailed in each subsequent appearance thus far.

In Yoshi's Woolly World, Nep-Enuts only appear in the course, Frozen Solid and Chilled, where they'll spit out Shy Guys at Yoshi. This was likely done to give Yoshi an infinite supply of Yarn Balls to stun the Nep-Enut with. They can also be frozen in that game, and a Ground Pound on one while it's frozen defeats it for good.

Trivia

 * The Super Mario World 2: Yoshi's Island box art and game cover shows Yoshi about to cross a gap with a Nep-Enut in hot pursuit.
 * Nep-Enut is an anagram of "Neptune", the Roman god of the sea.
 * Early screenshots of Paper Mario showed that at least one Nep-Enut was going to be in the game.