The entire concept of the Judas Breed. Man-eating Big Creepy-Crawlies is something scary by itself, but then you add in them evolving to vaguely mimic a human shape in the dark, allowing them to approach their preys close enough to strike.
All three movies, but Mimic 2 takes the cake with its head-stealing bug that wants to mate with a woman. And that doesn't die for three days after getting its head cut off.
It didn't appear on-screen, but concept art for the mimic-nymph carcass in the first film shows that it, too, had a face-mask it could fold over its own ... and it has facial features like a child's. Which implies those things hunt kids prior to their final molt to adult form.
