Harry Plopper (originally Spider-Pig) is Homer Simpson's pet pig. Homer first meets Plopper the Pig when he and Bart are in Krusty Burger.
Events leading up to the destruction of Springfield (caused by Spider Pig)
Plopper was adopted by Homer after he sees him featured in a Krusty Burger commercial filmed live at one of Springfield's Krusty Burger locations when he is about to be killed (he believes that animals wearing people's clothes shouldn't be killed). Although Marge wanted Homer to get rid of the pig because a warning seemingly crazy Abe Simpson had of "twisted tail" being one of the signs being trapped forever, Homer kept the pig.
Homer then played with Spider-Pig, particularly making it seem like he is crawling on the ceiling to spoof Spider-Man and leaving pig tracks all over the place. Homer puts his droppings in a giant silo that reads "pig crap" in the back yard. After Marge told Homer to dispose of the waste properly, and said that he could take Spider-Pig with him, Homer corrected her and told her that he renamed Spider-Pig to Harry Plopper, which is a spoof of Harry Potter.
Homer dropped his silo of droppings in Lake Springfield. This caused the lake's water to become mutagenic, leading to Springfield getting covered by a dome and an angry mob going after the Simpsons. Later while the Simpson family was escaping from their house during the mob by walking across a wood board leading to the Flanders' house, Plopper pushed the board making the Simpsons fall from it. He fulfilled part of Grampa Simpson's prophetic vision because of his twisted tail. He was seen being placed as a present from one of Cletus' relatives at Bart and Mary's wedding.
Plopper eventually became so famous, he was put on the Springfield Wall of Fame.