"The Losers Club" first realizes Pennywise's presence because of Bill's little brother, Georgie. King stated in a 2013 interview that he came up with the idea for Pennywise after asking himself what children feared "more than anything else in the world", and feeling that the answer was clowns.
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.
It's primary goal is to feed on humans, generally preferring children over adults since they were easier to scare and manipulate.
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.
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.
“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.
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.
Pennywise slumbered beneath the Earth for millions of years, awaiting the arrival of Mankind. When the town of Derry was built in 1715, IT awoke and began a cycle of feeding on the fears of the people of Stephen King's Derry and then resuming hibernation for cycles of 27 to 30 years.
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.
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.
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.
Though it is just before the next twenty-seven year IT cycle, this is our first glimpse of Pennywise in IT Chapter One. In the opening scene of the film, young Georgie Denbrough playing with a paper boat made for him by his brother Bill. He loses the boat down the sewer and faces Pennywise when he tries to get it back.
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.
The Losers moulded it into the form of a spider because that was all their puny human minds could perceive it as, but it wasn't really the spider. In the same way that a spider cannot truly understand what a human being is.
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 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 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 character is an ancient, trans-dimensional evil entity which preys upon the children (and sometimes adults) of Derry, Maine, roughly every 27 years, using a variety of powers that include the ability to shapeshift, manipulate reality, and go unnoticed by adults.
Both being interdimensional entities, their powers are at the same level, meaning that the turtle could kill IT if it wanted to. It makes sense that IT is afraid of Maturin – but the viewers and readers also have genuine grounds for cosmic terror.
Another explanation is connected to the Turtle and the cosmic side of IT and Stephen King's Macroverse, suggesting the Losers as the “chosen ones” that were guided and thus protected to an extent by Maturin, the only creature IT is afraid of.
The reveal of Pennywise's pregnancy may have been an essential moment in the It book ending, but there's a simple reason it was never shown on the screen.
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.
In Stephen King's book, an ancient turtle named Mataurin, who created the universe, gives those kids, known collectively as the Losers Club, the information to defeat It during a psychic ritual.