Nep-Enut did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Nep-Enuts (also spelled Nep-enuts) are large, blue Blarggs that appear in the Yoshi series. First appearing in Super Mario World 2: Yoshi's Island, Nep-Enuts have a near-identical behavior to Gargantua Blarggs; they sneak around through the water and, when Yoshi is nearby, will rise out of the water and attempt to attack him. Because of their large size, going past them is very difficult, and a Yoshi Egg is in most cases essential for passage as throwing one at a Nep-Enut will stun it. It then sinks back into the water and will continue to follow Yoshi, rising out of the water again.
Nep-Enuts later appear in other Yoshi games, including Yoshi's Island DS and Yoshi's New Island. In Yoshi's Woolly World however, Nep-Enuts have the added ability of spitting out Shy Guys, possibly to give Yoshi an infinite supply of yarn balls to stun it with. The only way to defeat a Nep-Enut is by freezing it with an Ice Watermelon, then using a ground pound to shatter it to pieces.