heh this is a client bug. If you leave the death screen up, the game constantly shows monsters spawning around you that are not actually there.
That scared me for a moment. If you hadn't told me the cause, I might've never gone into the frontier again!
Oh, so they're the ghosts of all the mobs that have been killed there, and since he's dead too, he can see them. :O
So this happened because I fell asleep while playing. I was breeding sheep for their heads. I just realized earlier today that I fell asleep... while counting sheep. How cliche is that?