It is a shape-shifting creature known as a Glamour, who is billions of years old. Although It lived on planet Earth for many years, It originated in a void/dimension outside the regions of space known as the Macroverse.
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.
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.
It is the main antagonist of Stephen King's It novel and movie adaptations. It is portrayed to be a shapeshifting and malevolent entity millions and possibly billions of years old, and with no exact gender or solid identity; thus why it is called It.
As Steve says, the creature fell to earth as a kind of a meteor during a prehistoric era, which would make it a few million years old.
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.
Pennywise isn't bad per se. I have no idea what It fed on for the millions of years before humans came to Derry, but you have to realize Pennywise is a creature. When he takes kids he is only doing it for food he isn't doing it out of malice or evil intent. That's why he only takes enough kids to satisfy his hunger.
Film: At the house on Neibolt Street, the kids basically do a bunch of damage by impaling It with rods. It looks like they get these weapons from the rusty wrought-iron fence of the house itself. Down in the sewers, during their final encounter, it's much of the same.
Still, the film gives viewers a pretty good sense of who Bill (Jaeden Lieberher), Beverly (Sophia Lillis), Ben (Jeremy Ray Taylor), Richie (Stranger Things star Finn Wolfhard), Eddie (Jack Dylan Grazer ), Stanley (Wyatt Oleff), and Mike (Chosen Jacobs) are.
Kersh is Pennywise's daughter.
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.
It Is Actually An Ancient Cosmic Force Of Destruction
It is the real monster behind the monster. If you want to get technical, It is an alien, but It's origin goes well beyond that. It is actually an ancient cosmic deity. A god of destruction, not just of our world, but of multiple worlds and even multiple universes.
Throughout the book, It is generally referred to as male, usually appearing as Pennywise. The Losers come to believe It may be female after seeing it in the form of a monstrous giant spider that lays eggs.
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.
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.
According to It, when humans got scared, "all the chemicals of fear flooded the body and salted the meat". This is why he prefers to feast on children -- their fears are simple, pure, and powerful compared to the complex, pathological fears of adults. Basically, children are delicious.
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.
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.
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.
In the movie, Pennywise says “They all float down here. You'll float, too”, referring to the floating bodies of the many victims that have been dragged to the sewers and… well, you know the rest.
For those unversed in the Kingsian universe, Pennywise is the evil antagonist, an ancient cosmic evil which preys upon the children of Derry, Maine, roughly every 25 years, using a variety of powers that include the ability to shapeshift, manipulate, and go unnoticed by adults.
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.
She is the daughter of Pennywise and was brought up by him and incorporated into all of his evil ideas. She's a part of the whole thing. It's a transformation.”
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).