The true genius of IT, at least as far as being scary, is that it can become basically anything, with almost no real limitations. IT can imitate humans, transform into animals, it could probably become a chair and wait for someone to sit down if it so chose.
Pennywise is only one of Its forms. In the film, we also see It become a mummy, Beverly's dad, Mike's burning parents, the creepy painting lady, a decapitated boy, a leper, and Georgie, and in the book, he takes many more forms, most famously, the classic Universal monsters.
He took the form of a clown most frequently, Mr. Bob Gray or Pennywise, but his true form is an ancient eldritch entity from another universe who landed in the town that would become Derry by way of an asteroid and first awoke in 1715.
This creature fed on humans and their fears, especially children as their fears are less complex, and used its shapeshifting abilities to catch its victims. IT could take any form, but its preferred one was Pennywise, the Dancing Clown.
Aside from Pennywise the Dancing Clown, It has transformed into Georgie Denborough, Dorsey Corcoran, The Creature from the Black Lagoon, Betty Ripsom, a giant bird, werewolf, leper, mummy, and piranhas.
Literature. In the novel, It is a shapeshifting monster who usually takes the form of Pennywise the Dancing Clown, originating in a void containing and surrounding the Universe—a place referred to in the novel as the "Macroverse".
Kersh is Pennywise's daughter.
Immortality: Its earthly avatar has existed on Earth for thousands if not millions of years. Its true form has existed even longer in the Macroverse (a void outside of time and space that surrounds our own universe).
Stephen King's 'IT' introduced readers to a one-of-a-kind creature that can take any form, the most common one being Pennywise the Dancing Clown, and as menacing as it is, this creature has one big enemy it's truly scared of: Maturin, the turtle.
Muschietti appears to be implying Pennywise was a human familiar who It corrupted and then enjoyed so much It incorporated his shape into its file cabinet of monsters.
It Is Actually An Ancient Cosmic Force Of Destruction
It is actually an ancient cosmic deity. A god of destruction, not just of our world, but of multiple worlds and even multiple universes.
Pennywise's origins are briefly explored in both the book and It: Chapter Two, where it's revealed that he is a being that crash-landed on Earth from another dimension hundreds of years ago.
Throughout the summer and their trials against Pennywise and the school bullies, the Losers realize that none of the adults in Derry can actually see Pennywise or the horrors he is committing against the children of Derry.
As well as having his right arm bitten off, Georgie is trying to crawl away but he was dragged into the sewers and devoured by Pennywise, with an unnamed older woman and her cat being the sole witnesses of the horrific scene, including before Georgie lost his arm.
That is, when Pennywise morphs into a werewolf, he is subject to the same weaknesses as a werewolf, including silver slugs. In both the original novel and made-for-television adaptation, Pennywise is weakened using silver bullets melted down from earrings.
Originally Answered: Why did Pennywise start on Chapter Two with attacking an adult? Pennywise thrives on fear. By killing him, Pennywise gets the town starting to fear others.
Down in the sewers, during their final encounter, it's much of the same. Mike brings the gun he uses to kill livestock, and the rest of the Losers come prepared with more rods. Beverly deals the final blow by putting one straight down It's throat.
The winner is Lucifer Morningstar and maybe quite easily. Lucifer as his name suggests is The Devil of the DC Universe, he is by far one of the most powerful beings to exist. He is the first and most beautiful and the most powerful of all the angels in existence.
IT thrives on chaos; an exact 27-year pattern is way too predictable for a being of pure evil. Rather, IT wakes up roughly every three decades, and stays away for a different period of time.
In their final battle, the Losers' Club was able to kill Pennywise by reducing him to a child-like form as they overcame their fears.
The concept of clowns has been traced back to the Fifth Dynasty of Egypt, but the modern circus clown developed in the 19th century, so by the time King's novel is set (the first part of it), Pennywise was around 200 years old.
This is one of the most interesting Hollywood couplings we learned about in quite some time. Apparently, Pennywise and the Babadook are dating. In fact, they're in a long term committed relationship.
Throughout Stephen King's It, Pennywise is referred to as a male, but author Stephen King pulled a fast one on readers in the It book ending by revealing the creature's true form was a pregnant spider, implying that it is, in fact, biologically female.