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.
Martin Morgan Pennwise is (biologically) asexual and has the ability to reproduce. Therefore even though King describes pennywise as a female further into the book, 'IT' is (technically) neither male nor female.
It was said on page 1361 that the Spider being both female and pregnant was a symbolic interperetation. Its not literal, but a representation of whatever equivalent but inconceivable reproductive capability It actually has. Pennywise didn't become pregnant.
Regardless, she's one nasty beast and not alone due to the egg sack she's carrying. Unfortunately, King's imagination ran a little wilder than alluded to in the movies or earlier in the books, and the consequence of that is a pregnant Pennywise.
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.
However, despite all that, there's one thing IT doesn't feature, and that's Pennywise devouring a baby. King doesn't go quite that far, although babies aren't safe from violence in the book completely, thanks to Patrick Hockstetter.
It's clearly established in the It book ending that Pennywise most commonly appears to the Losers in the shape of a clown. As the creature is referred to as "he" in this form, it can be assumed that the clown is male.
And if his victim believes him to be powerless, he will become powerless. And so the group figures out that their best bet is, essentially, to bully the bully: They mock Pennywise, telling him he isn't scary. Humiliated, Pennywise begins to shrink, transforming into an almost Benjamin Button–like, melted clown-baby.
Whilst most assume that Pennywise is male it's actually a female and in the story, we discover that the creature is actually pregnant and has also laid several eggs, though it is killed before it can give birth and the lair is destroyed, presumably along with it's children.
He explains to Conan how in his childhood, he used the demonic smile to frighten his little brother after learning his older brother Gustav was also able to point his lip.
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.
Technically speaking, Pennywise is not actually what It is. Rather Pennywise is just one of many forms an extraterrestrial, godlike being takes in order to lure small children into the deadlights.
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.
If we're talking Pennywise per se, it is "male," as shown in the old b&w photographs of Derry, scenes in sewer, etc. But as others have said, Pennywise is merely one manifestation of IT as a larger entity, itself which is not inherently one specific sex or gender but whatever form of sex or gender IT prefers.
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.
In the books, Pennywise's true form is revealed to be a giant spider, a pregnant one, which might even make it female. Pennywise's lair is also strewn with webs and eggs, indicating that perhaps Pennywise was planning to expand to other locations, or at least, replace itself if the Losers defeated it.
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.
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.”
“I told Bill, 'I want it in both directions, I want you to send your eyes looking away and give a really unsettling look'. He said, 'I can do it', so he did it. What are the chances? It's one in a million and he was cast already.
[Stephen King's IT] Why doesn't Pennywise go after adults the way he goes after kids? Pennywise's deal is that he strikes fear into the hearts of others before devouring them, as essentially fattening them up before he feasts.
This would lead to one of the biggest Stephen King book controversies: after the Losers become lost in the sewers trying to make their way back to the surface, in IT Beverly decided to have sexual intercourse with all the boys from the Losers Club.
Finally, Pennywise is beaten into submission. He scurries away, utters the word "fear," and partially disintegrates before falling into the void. It's a powerful defeat of a powerful monster, and it's satisfaction enough were IT to remain a single film.
