one of the countless faces it uses on a regular basis. The monster that haunts Derry is a creature of habit. The clown disguise was adopted as a way to get close to children, not to scare them. It learns certain shapes and lazily uses them over and over again when it isn't pulling an image directly from the mind of a victim.
In short, It isn't a clown. It isn't Robert Gray. It isn't even really a spider. By the end of the book the importance of the name is zero. The only name that really applies is the one the Losers gave it, i.e. IT.