Harry Potter Theory: IT's Pennywise Is A BOGGART!

5. The Darkness

In the Wizarding World, Boggarts are particularly fond of inhabiting dark, confined spaces, such as in wardrobes or under beds or the cupboards under sinks and desks. They seek darkness for comfort.

In the same sort of way, Pennywise is presented as a bogeyman who lives underground within dark tunnels and whose stalking of children usually involves enclosed, dark spaces. He is found underneath the bleachers, in a storm drain, in a creepy hall of mirrors, in a library archive... These are all places a boggart would also find comforting. At two points in the IT double-header, he chooses to squeeze into an old fridge - it's not merely for effect, clearly, it's because he fits best there.