Anything that looks threatening. It's an evolutionary mechanism by which as primates we're sensitive to any danger that seems to be looming, if that's visual, auditory, olfactory, even conceptual, or whichever.
Creatures in horror films that have large glaring eyes are simply scary because they signal, in our animal brain, a predator may want to feed on us or abuse us in some way. Anything creepy or weird relates back to feeling in danger for our lives or wellbeing. All horror is in fact based on exploiting those rational fears, however it stretches the basic fear into finetuned creations, visuals and sounds which real roots are difficult to even identify as they've been so modified and processed.