Taken over by an extra-terrestrial force which landed as a meteor millions of years ago?
shareThe actual origin of IT is widely debated but it is likely that it is one of the primal demon elementals and are equivalent to Gan. If you are not knowledgable in Stephen King Universe ill give you a simplified version: IT is one of 6 gods.
Pennywise was likely the first candidate for form adoption IT has found after the native tribes moved away from the place.
pennywise is one of the forms of IT who is an alien.
shareThe extra-terrestrial force manifested itself as a spider monster. The spider monster can turn into anything and it can read minds, so it can turn into things which scares its victims. It likes to scare them because it makes their flesh taste nicer. (I guess the flight-or-fight hormones do this?)
It's not entirely clear if killing the spider form actually kills it entirely, or just expels it from the universe. So it's possible that the malevolent form still exists, but can't come back to Earht.