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 spit out Shy Guys. 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.