Boo! It’s a Witch
Okay, so can you buy this? Boo is actually the witch from Brave. The theory has it, that the witch and scarers from Monsters inc. magically disappear when they go through doorways. Maybe Sulley taught Boo how to do it too. Proof includes a carving of Sulley in the witch’s workshop carved hundreds of years before Monster’s Inc.
Which opens the door to characters traveling through time from one Disney movie to another. Why not.