The titular monster in IT's default form is Pennywise the Dancing Clown, but he had to get that identity from somewhere, probably a real person. Often when talking about Stephen King's IT, people will use the name Pennywise when talking about the monster, and while that's understandable, it's not really accurate. The movies don't really get into IT's origins much, but King's book makes clear that the shape-shifting creature is an alien that arrived on Earth thousands of years prior, and is billions of years old, having originated in a dimension outside of normal reality called the Macroverse.
In some ways, IT is a creature akin to a Lovecraftian eldritch abomination, in that its power to manipulate reality is almost limitless, and its true form isn't one able to be readily perceived by humans. Those that do gaze into its "Deadlights" are never left the same mentally either, similar to how gazing upon an "Old One" often causes the beholder to go mad from the revelation.
Basically, while IT most often assumes the guise of Pennywise the Clown, the truth of what IT is is so much more complex. However, considering how often IT transforms into creatures from pop culture in King's book, it stands to reason that a real dancing clown must've inspired the monster's go-to form.
IT Theory: IT Killed the Real Pennywise the Clown
Stephen King's IT novel doesn't really address the inspiration for Pennywise's look and mannerisms, outside of the idea that a performing clown would be perceived as non-threatening, especially by children. A scene in IT Chapter Two though suggests a more specific take on IT's Pennywise identity. When Beverly encounters Mrs. Kersh at her childhood home, the old lady turns out to be IT in disguise, but before that, Kersh regales Beverly with a long story about her father joining the circus, with photos on the wall featuring what appears to be Pennywise without his clown makeup and costume. Beverly then briefly encounters Pennywise sitting in a chair and wearing only partial clown makeup, played of course by Bill Skarsgard.
While that could've all been mind games by IT against Beverly, it would make perfect sense that at some point in Derry's long history, IT developed a fascination with an actual Pennywise the Dancing Clown entertainer, and decided to assume his identity in order to travel among Derry's residents more openly and freely. If there's one thing IT seems fond of, it's showmanship, as after all, strong emotions, especially fear, make ITs victims taste better. That said, IT likely would've dispatched of the real-life Pennywise, not wanting to share ITs new favorite persona. IT normally preys on children, but is perfectly capable of targeting adults when it chooses to, so this wouldn't have been hard to accomplish.