Stephen King loves to pepper his books with references to his other works. He's like Pixar, if every character was Sid, and this rich tradition is honored in 2017's Dark Tower film.
As Jake enters Mid-World, he walks through an abandoned carnival. At one point, he comes across a statue of a severed hand holding three balloons. And what creepy Stephen King character loves his balloons? Yep, that would be Pennywise, who can take any form It feels will scare a child enough to make them edible, but who largely prefers to be the scariest clown this side of the Joker. And just in case you weren't sure, above Jake stands a large, dark, crumbling sign that clearly says PENNYWISE. In addition to being pretty freaky, this egg actually makes narrative sense. Pennywise can be anything and, as an interdimensional entity, go anywhere. So why wouldn't he chill in Mid-World, looking for a meal or two? After all, they float over there too.