There was a human Pennywise the Dancing Clown, but IT just takes his form. In the movie canon, Pennywise the Dancing Clown was a real person whose image IT decided was a terrifying enough basis for his child-scaring campaign.
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".
inside them… inside Me…” So in a nutshell, Pennywise/It's true form is floating balls of light out in space, and if you look at them in their true form, your mind will live eternally in It's thrall (this is what happens to Bill's wife Audra in the book).
In the novel, It's origins are nebulous. 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.
His name was Robert Gray, better known as Bob Gray, better known as Pennywise the Dancing Clown." This is also the name It uses to introduce itself to Georgie, Bill's brother, in the novel. The surname Gray has appeared before in other King novels, notably Dreamcatcher, published in 2001.
“It” is an evil entity that usually takes the form of a clown named Pennywise (played by Bill Skarsgård) and returns to Derry, Maine every 27 years to terrorize the town. The red balloon is a harbinger that It is watching you.
The 27-year cycle refers to the extended interval between waking periods of the extra-dimensional shapeshifter in the novel IT, as well as the interval between occurences of The Troubles (outbreaks of increased supernatural activity) in the television series Haven.
King decided for IT to predominantly take the shape of Pennywise the Dancing Clown because he believes "clowns scare children more than anything else in the world." IT influences the adults of Derry to passively ignore it and not interfere with his attacks on Derry's children.
Related: What Does Pennywise Really Look Like In IT? IT arrived on Earth through an event similar to an asteroid impact, landing in what would later become Derry, Maine. Once there, IT adopted its usual pattern of hibernation that lasted between 27 and 30 years, awakening to kill and eat and then going back to sleep.
Every 27 years, a shape-shifting entity known as IT crawls out of the sewers to prey on the children of Derry, Maine. IT plasters himself with an unnatural grin, takes on the form of Pennywise the Dancing Clown, and wreaks havoc.
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.
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.
It feasts on the flesh of humans simply because our fears are easy to manifest and they make us taste better. According to It, when humans got scared, "all the chemicals of fear flooded the body and salted the meat".
In one scene, Jessica Chastain's Beverly sees a vision of Pennywise the clown before he puts on his white paint and we finally get a glimpse of Bill Skarsgård's actual face. He's the man behind the terrifying evil entity that's giving an entire new generation a clown phobia.
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.
The spider-clown shrinks as the Losers hurl taunts at It, until it's tiny and weak enough that they pluck out its heart and squash it into nothingness. In the end, they defeat Pennywise by, uh, making him feel really bad about himself.
The Deadlights are orange writhing lights that exist in Todash Darkness. Pennywise uses its Deadlights to break a person's mind because one look at the Deadlights will make a person go insane due to it not being able to be comprehended by a human mind.
The simplest and most obvious interpretation is that all the talk of floating is a reference to the fact that Pennywise (aka the titular "It") murders his victims and drags them down to the town's sewer system where he dwells, which is full of water. And what do dead bodies do in water? That's right — they float.
The cinematic adaptation of Stephen King's IT Chapter Two depicted the satisfying death of Pennywise while subtly hearkening back to Pennywise's first 1988 victim, Bill's brother Georgie.
The kids at the heart of the It call themselves the Losers' Club, but even among the seven of them, there have to be winners and, well, losers.
It's weaknesses are courage and heart. For the sake of spoilers, I won't go too much into the Ritual of Chüd, but suffice it to say that if you want to defeat It, you've got to have the two traits listed above.
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.
Being such a powerful being, Pennywise clearly sensed that the yellow coloring was unsettling to Georgie, so he changed his eyes to blue on the fly as a way to make Georgie associate him with family and safety. This made Georgie all the easier to lure in to his bloody demise.