From one of the worlds that doesn't have it? That's fine.

[he nods, wondering how to explain.]

It'd be easier to learn about it from a book. How it started, I mean. At least on some worlds. Basically, at the end of October, there's a whole lot of scary stuff that happens. So if you see monsters that aren't Heartless or Heartless-like, chances are they won't be anything to worry about unless they're actually attacking people.