Goth Empress

If you read the book, Stephen King specifically says that IT (the shape-shifting monster also known as Pennywise) is female and pregnant. This is why the Losers’ Club has to come back and finish what they started when they were kids. 

However, IT chooses primarily male forms or genderless forms to kill and terrorize the children with. 

So, IT specifically chooses to fluctuate between female, male, and genderless presentations, making IT genderfluid.