I think the scariest thing about clowns is not that they are monsters, although Pennywise is, but just dudes in suits and make-up, so they are more reality based than say Wolfman.
If you’re anything like me, you find clowns to be scary as hell. You also love being scared so you seek out those things that make you want to pee your pants. I know exactly when my clown fear kicked in. The year was 1990, and Stephen King’s epic novel IT becomes an even more epic mini-series. I was 7 years old, and had a Stephen King fan for a mother, so yes, I was allowed to watch it. Needless to say for year I was convinced that Pennywise and Freddy lived in my closet.