Based on the novel by the king of horror - Stephen King, the movie intrepretation of 'IT' certainly did the book justice. But there was one thing they left out, and that is the origins of Pennywise, the Dancing Clown (and the Ritual of Chüd).
But according to the actor who plays the serial killer clown that preys particularly on kids - the movie almost covered the backstory of Pennywise.
Bill Skarsgård told the Variety in a recent Podcast, about a scene that did not make it to the final cut of IT: Chapter one. "There's a scene that we shot that was a flashback scene from the 1600s, where Pennywise was — before Pennywise," says Skarsgård
"We shot it for additional shooting, and the scene turned out really, really disturbing... and I'm not the clown, I look more like myself... I don't want to spoil what the scene is, because we might use, it but it's a very disturbing scene, sort of a backstory for what It is, and where Pennywise came from." he adds, "that might be something really worth exploring in the second one as well."
Skarsgård further said that the deleted scene takes place around the time when Derry started to get populated, where IT has been dormant for thousands of years.
There are already talks about the second chapter of the movie, with the Losers Club turned into adults. Maybe we will get to see more of the creep murderous clown's backstory in the second movie.