Does that concept sound familiar to you? It may be because you've seen Harry Potter and the Prisoner of Azkaban.
Fans on Twitter have put together that Pennywise is pretty much a boggart, a type of being that exists within the Harry Potter universe. The creature first makes its appearance in the Prisoner of Azkaban, where Professor Remus Lupin brings one to his Defense Against the Dark Arts class. He releases the boggart, who turns into a variety of things depending on which Hogwarts student is staring at it. Unsurprisingly, Ron is forced to confront a "spider" — or, rather, the boggart masquerading as one.
Other people within the wizarding universe have had their own boggart experiences as well. Mrs. Weasley sees the boggart as the dead bodies of her loved ones. Neville sees Professor Snape. Hermione sees Professor McGonagall telling her that she is an academic failure. The way to fight the monster is to do the charm "riddikulus," which ultimately turns the monster (or whatever else you are seeing) into something funny and no longer scary.
As for the monster in It, well... things are a little more complicated.
Fans of both It and Harry Potter can't get over the similarities.
Okay, so It and Harry Potter don't exactly share the same universe, but maybe J.K. Rowling was a little bit inspired by King's creepy work. Fortunately, she made the boggart a little easier to defeat than the child-eating Pennywise.