Pennywise isn't a spider. It isn't a clown, a werewolf, a vampire, a mummy or a thousand other things besides. Its name isn't really Pennywise or Robert Gray for that matter. The final shape that it took in the cave was impressed upon it by the Losers. The monster itself never "dresses for dinner" in its own Lair. The spider-like shape was merely the closest thing their minds could make sense of from the metaphoric nature of a spiritual entity. That doesn't mean it looks spider-like and thus their mind agreed upon a spider. It means the creature's ephemeral nature embodies certain behaviors and relationships between it and people. It acts rather like a spider and thus in the end they made it look like one. Does that make sense? You could consider all of Derry, its personal killing ground, as no more than a giant web where it resides and feeds.
We all know that the monster's true form is nothing but maddening lights outside of reality, always seeking a way in. Sometimes it finds a pinhole in that cosmic barrier and through such openings a tendril will ooze through. The irony is that since it cannot come through entirely, the bit of the Deadlights that slips in must conform, somewhat, to reality and be quasi-physical. Even how it thinks and perceives itself is a reflection of the reality it has invaded. Think about when it was hurtling Bill toward the larger, truer part of itself. The further they moved away from reality, the more insane, chaotic and burbling the thing became. In other words, the Deadlights outside of reality as a whole is not Pennywise or even aware of itself in the same way. It is primordial chaos out there. Not only is its physical from created by reflecting our reality and understanding, its own understanding of itself is also shaped and created only as it comes in. The part of the Deadlights that slips through manifests as an avatar which is unique and individual, created by the world, location, rules of reality, and the people themselves which it reflects.
