We saw this a lot in the original miniseries, as the inter-dimensional menace not only shifted into the guise other people, but we learned that his clown form was a disguise as well.
Now before you write off Pennywise as a Freddy Krueger rip-off, there’s a crucial difference that separates It’s clown menace from the clawed kiddie diddler: Pennywise doesn’t depend on you falling asleep.
In fact, his specialty is the waking nightmare, as he can appear as anything, or even merely as himself, in any place possible.
Though it’s still to be determined whether or not Andres Muschietti decided to adapt the gigantic space spider of the original miniseries, or to come up with a brand new final form for Pennywise.