IT takes the form of Pennywise the Dancing Clown because King believes “clowns scare children more than anything else in the world.”
Though IT likes to manifest itself as a male clown named Pennywise, in the novel IT also takes the form of an enormous female spider.
IT would prey on the children of Derry because children’s fears are easier to harvest and then manipulate into physical form.
Pennywise the Clown largely remains a mystery to the Losers and likewise to many in the audience who are not versed in King’s seminal novel and experienced IT for the first time this weekend.