What exactly is the monster in it?
				  
				  The Monster in "It" Explained
In Stephen King's novel "It," as well as its film adaptations, the monster is an ancient, cosmic entity known simply as It. This creature is a shape-shifter that most often appears as Pennywise the Dancing Clown, but can transform into whatever its victims fear most.
Nature and Origin
- Origin: It is a primordial being from a void outside the universe, referred to as the "Macroverse."
 - True Form: Its real form exists outside of human perception, described as "deadlights," which are incomprehensible beams of energy or light.
 - Purpose: It feeds on fear, especially the fear of children, and awakens roughly every 27 years to prey on the town of Derry, Maine.
 
Manifestations
- Pennywise the Clown: Its most iconic and frequent guise, designed to lure children.
 - Other Forms: It can appear as lepers, werewolves, mummies, or any other creature tailored to an individual's worst fears.
 
Summary
In essence, the monster in "It" is a shape-shifting, fear-eating, ancient cosmic entity, most recognized as Pennywise the Clown but capable of manifesting as any terror its prey imagines.