Though sometimes, "just another creepy monster" - something new that the PCs (and players!) haven't seen or heard of before - is exactly what's required at the time. As in:
DM rolls give a random encounter, DM quickly thinks to self: "Hmmm. Do I throw yet another monster at 'em that they've seen and beaten a hundred times before, or do I dream up something brand new right now that suits the surroundings? Yeah, let's go for something new..."
And it doesn't even have to have anything to do with someone's sister in order to make it a) a threat and b) interesting, if the DM does it right.