What is it about a monster you are most scared of?

I'm writing a horror novel and bestiary and I have drawn up some terrifying creature concepts ( They makes Dead Space's Necromorphs look like a kittens.) I'm just wanting to know what you guys personally find scary. example: H.R. Giger gave the Alien no eyes because he discovered people are more scared of something they can look at but cant tell if its looking back and the fact that they are being watched or stalked by something that doesn't even have eyes freaks people out. So, if there was an embodiment of all your fears standing before you ready to slice you to bits, what features would it have? tail or no tail? teeth or no teeth? tentacles? bones? No bottom jaw (I'm talking to you zombies) go nuts, tell me what would scare the living daylights out of you. I would really appreciate it :) thank you very much.