It is probably my favorite book. I'm dying to see the movie but social anxiety tends to keep me away from movie theatres.
And, yes, I just got done with rereading the book and Pennywise is some ancient Eldritch abomination type thing. It can take any form but it's preferred form is that of a clown. It's true form is revealed to be a giant spider. But that is only because it's the closest thing to what it really looks like that the human mind can comprehend. Those who see it as it really is or looks into it's "deadlights" is driven insane or goes catatonic. So, yeah, an alligator would be about as much a threat to Pennywise as a baby cockroach would be to a human.