The shapeshifting abilities of Pennywise provide some of the most memorable scares of It: Chapter One, showing him in numerous menacing guises throughout the film in addition to his default appearance as a sinister 18th century circus clown. According to director Andy Muschietti though, it was considered at one point for one of those transformations to see him turn into A Nightmare On Elm Street‘s Freddy Krueger.
The movie inspiration of Pennywise’s illusions stems back to the original novel, where he variously takes the form of Dracula, the Mummy, the Creature From the Black Lagoon, the Wolf Man and Frankenstein’s Monster. This was partly due to the adolescent-focused parts of the book taking place in the ‘50s, whose children would have been terrified by the Universal Monsters, and partly due to Stephen King’s love of dated pop culture reference points.
The idea of utilizing Freddy Krueger was an extension of this, with the setting of the first film being updated to the late ‘80s when the iconic slasher franchise was at the height of its popularity and imagery of its villainous fear demon was ubiquitous, to the extent that A Nightmare On Elm Street 5 is briefly seen advertised on a cinema marquee.
It was ultimately decided to abandon the idea though, with Muschietti deciding that the use of the horror legend would have been a distraction, and instead chose to focus on fears more primal and personal to terrorize the children, such as mocking loved ones twisted in death and the likes of a headless incinerated corpse and a rotting leper.
Leaving out Freddy was probably for the best in the long run, too, since while bringing him in for a cameo (and probably having Robert Englund play him again) would have provoked an immediate positive reaction from horror fans, it would have also raised the question of how well the film was able to stand on its own merits if it needed to invoke another, far more famous series to maintain its fear factor. After all, not relying on the use of genre tropes was one of the things that made It: Chapter One such a success in the first place.