Nep-Enut

Nep-Enut is a type of blue, water-based 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, and Yoshi's New Island. 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 to hide (and only its eyebrows will remain visible). If Yoshi jumps over the giant monster, Nep-Enut will give chase. Nep-Enuts are much bigger and more detailed in Yoshi's Island DS than in Super Mario World 2: Yoshi's Island.

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.