I was going to say walrus but reading the preamble it says there's a knock first and that's really throwing me. Walruses can't knock, right? So if you heard a knock and saw a walrus you'd assume it's a prank, which is less surprising bit still surprising but not as surprising as not hearing a knock and seeing a walrus. But if you hear a knock and see a fairy that's also more surprising in some ways because how can the fairy knock loud enough that I'd recognise it as a knock and not a tap? Unless the walrus is a sentient walrus knows humans knock on doors to announce themselves, which makes it even more surprising if it were that kind of walrus but it doesn't say that it's a special walrus. So I'm saying fairy.