Pennywise transforms into many things in IT Chapter One, but he came close to becoming another Warner Bros. horror icon, Freddy Krueger. It's not an aspect that ended up playing a part in the IT screen adaptations - outside of Richie's brief werewolf encounter in the miniseries - but Stephen King's book sees the titular shape-shifting monster take on the guise of several well-known creatures from pop culture, including Dracula, Frankenstein's monster, and even the shark from Jaws.
This made sense, as the Losers' Club were kids in the 1950s, and grew up watching and being frightened by the legendary Universal monsters. The Jaws appearance is likewise appropriate, coming later in the timeline when IT is preying on kids in the 1980s, many of whom had seen the 1975 cultural phenomenon. Of course, accomplishing such transformations would've always been harder to pull off onscreen, where different studios own different movie monsters, and nobody really wants to have to pay a rival to borrow one of their characters.
That said, there's some horror characters that IT and IT Chapter Two director Andy Muschietti and writer Gary Dauberman could've used with minimal fuss, those being the characters already owned by Warner Bros. When it comes to horror, Warner Bros.' most iconic property outside of IT itself is A Nightmare on Elm Street, starring scarred slasher Freddy Krueger. It turns out that Freddy was indeed considered for a cameo in IT.
Freddy Krueger Almost Appeared in IT Chapter One
According to Andy Muschietti himself, the director did briefly consider having IT transform into Freddy Krueger and menace the Losers' Club. Such an appearance would've made perfect sense, as the childhood portion of the story is set in 1988 and 1989, when the A Nightmare on Elm Street franchise was hitting its peak of popularity. Freddy starred in a new film during both years, and A Nightmare on Elm Street 5 is even seen on a Derry theater marquee in IT Chapter One. Muschietti really enjoyed how King used the Universal monsters against the Losers in the book, and that's why he considered incorporating Freddy in the same manner.
Unfortunately for Freddy fans, Muschietti eventually decided not to have IT become The Springwood Slasher, feeling that an appearance by such an iconic villain would just distract audiences from the story. Instead, Muschietti chose to focus on IT's already quite terrifying default form of Pennywise the Clown, and save transformations for fears more personal to the Losers' Club, such as Eddie's leper or Bill's little brother Georgie. If Freddy had been part of the IT movie though, one wonders if actor Robert Englund could've been lured out of retirement for the occasion.