Pennywise, as in the name, is inspired from the real life IT novel written by Stephen King. The novel and the film based on it had a cult following during the 1980s, hence the legend of Pennywise began haunting some lesser known myth hunters in the game.
The myth hunters claiming to have sighted Pennywise interlinked bizarre connections between Pennywise and the Ice Cream Man, stating that both share similarities and may be the reason of scaring away adolescents in the game, thus giving a reason of the exclusion of minors in the game. However, the tales of Pennywise can easily be debunked with the help of technical files. The rumors of Pennywise and nearby Circus wheel can be a reference, but Pennywise doesn't physically exist in the game.