404 Not Found Pig is a pig that can make birds go backwards (with the exception of Blue Bird, Terence, and the Mighty Birds.) He is based on the 404 Not Found Error. When you kill him, he glitches out before popping.
His reappearance is Some Kinda Webbirds has some changes: He lost his "backwarder" ability, and is completely invisible. So, he would be killed by luck, but he can be appeared by a power-up named "Cache Recovery".