Pennywise The Dancing Clown

First appearance: 2017

A bunch of kids run afoul of some kind of horrible shapeshifting monster that lives in the sewer beneath there home town. The monster takes the form of what it's victims fear most favouring the look of a creepy clown, and surfaces every 27 years to feed.
Movie based on the Stephen King novel but shifts the story 27 years into the future.

Pennywise The Dancing Clown

First appearance: 1990

A bunch of kids run afoul of a giant spider from outer space (possibly from another dimension) who wakes up every 27 years to feed. The space monster takes on the forms of things it's victims are scared of. This doesn't stop the kids from defeating the spider monster.
Unfortunately for the kids they didn't kill the space monster so IT comes back 27 years later to annoy the now adults to death.
90s TV Miniseries Based on a Stephen King novel and for some reason very popular even though the monster is more irritating than frightening.