IT

IT is the titular main antagonist of Stephen King's novel IT and its film adaptations IT: Chapter One and IT: Chapter Two. It is a demon from another world that haunts the town of Derry, Maine, often feasting upon the fears of children every 27 years. IT is known for shapeshifting into the fears of its prey, its most famous and default form being Pennywise the Dancing Clown.

Villain Pub: Penny For Your Fears
After Pennywise is defeated by the Losers, it crash-lands into a 1980's-themed Villain Pub. There, it meets the Jack Nicholson Joker, Freddy Krueger, Emperor Palpatine, and various horror-themed 1980's villains as it recaps and laments on its reign of terror over Derry.

How IT Should Have Ended
IT, as Pennywise, is first seen confronting Georgie after he loses his boat. As it emerges from the sewers, Georgie flees, screaming, "Stranger danger!" as it grumbles in defeat.

In the next scene, while not seen, IT attempts to convince Beverly to approach a sink. She refuses, much to its displeasure.

In another scene, Pennywise hijacks the Losers' presentation of it, but Beverly manages to disconnect the projector. Pennywise then appears and reminds the Losers that it is magic, much to their horror.

When the Losers arrive at the Neibolt House, Bill rallies the Losers to stick together. However, Bill later stumbles upon IT feasting on Stan, and repeats his oath to stick together in exasperation.

During the climactic showdown, Pennywise captures Bill and vows to feast on the Losers' flesh and fears, but Batman arrives and frees Bill. The Losers then realize that they can defeat IT by summoning the most powerful characters to help them, with the Teenage Mutant Ninja Turtles and King Kong attacking IT and the Ghostbusters capturing IT.

In the final skit, IT uses the Deadlights on the Losers.

How Avengers: Infinity War Should Have Ended
IT is one of the many villains killed by Thor's snap.

How Deadpool 2 Should Have Ended
IT, as Pennywise, is one of those attending the auditions for Deadpool's X-Force.

Powers and abilities

 * Shapeshifting: IT's most iconic ability is shapeshifting into the worst fears of its prey. It primarily assumes the Pennywise identity, playing into the common fear of coulrophobia, but can also assume other guises, such as Judith the oil painting lady in the IT: Chapter One HISHE and Darth Vader joining the light side in "Penny for your Fears."
 * Deadlights IT's true form is comprised of the Deadlights, a deadly form of energy hailing from the Macroverse. The Deadlights can instantly kill anyone who looks at them, even at a brief glance. IT unleashes the Deadlights on the Losers in the IT: Chapter One HISHE.

Appearances
Villain Pub-Penny For Your Fears (first appearance)

How IT Should Have Ended

How Avengers: Infinity War Should Have Ended (cameo and death)

How Deadpool 2 Should Have Ended

Trivia

 * IT, as Pennywise, is played by Bill Skarsgård in IT: Chapter One and IT: Chapter Two.
 * This is referenced in the Deadpool 2 HISHE, where Pennywise's presence in the X-Force alludes to Skarsgård playing Zeitgeist in Deadpool 2.