What kinds of horror creatures are scariest to you?
I write post-apocalyptic action/survival horror. I'd like to know what characteristics of a monster disturb and otherwise scare people the most. I'm talking strictly physical creatures, in this case.
Personally, I think something that makes you one of them, or a part of it, really gets to me. Like the headcrabs from Half-Life, or necromorphs from Dead Space. Zombies are technically an example, but that leads me to another point.
Traditional zombies aren't really scary. Of course, if one shambled into your room irl, that would get your heart going. They're not scary in fiction to me, and I think part of that is because we understand them. You know their weaknesses, and you know that a bite makes you one of them.
I think understanding something makes it less scary. Knowing nothing about a creature, and having no idea how to deal with it, makes it much more alarming. It adds an extra layer of confusion, and a need for more time to think about what to do. Furthermore, I think creature types having names makes them less scary. Like say, the special zombie types in Left 4 Dead (charger, tank, smoker, etc.) The fact that they're familiar enough to have names makes them less scary. I think what's really scary is something so bizarre and foreign that you don't even know what to call it.
Directly mentioning elements of creatures we know, like real animals, make them less scary too. I try to avoid that. Like saying something has a canine-like body makes it seem even just slightly less scary to me. We know what dogs are, so we've seen something vaguely like it.
However, it really bothers me when something has some humanlike qualities, be it visually or audibly. People are naturally creeped out by things that look human but aren't, like dolls and mannequins. Anything having humalike flesh gets me. One monster I created makes a sound when it notices prey that I described as someone gasping for air after surfacing water, having been a second from drowning. Creeped myself out with that one.
Side note. I'm immediately creeped out by too many eyes and legs because of my spider phobia. Tentacles are also unnerving to me.