The sheet phantom is a ghost... that materializes as a sheet. I think it's based off the old "hey, it's just a guy with a sheet over him pretending to be a ghost" joke. But the idea of a haunted bedsheet just isn't cool at any speed.
Bear: Bet you guys can't guess what I'm going as.
Franklin, Beaver, Fox and Goose: A ghost.
Bear: Ah, how'd you know?
Franklin: (giggles) 'Cause that's what you go as every year. But it's a really good costume, Bear.
— "Franklin's Halloween" from Franklin
