A "winged freak" from my current world.
Sleek skin, seeing the organs and muscles held within. Backward joints, moving with a terrible gait unlike any creature of the day. Blank space where the eyes should be, thin bat-like wings, this thing gives plenty reasons to fear the night. Welcome to the Silent Hills
Seeing one stalking an adventurer through the woods at night would be terrifying.
I mentioned it once before in another thread, but while they aren't quite Cthulhu-esque horror, they are definitely things that should not be. They can also be pretty damn large (varying quite wildly - a random one I have in this folder of extracted procedural raws I downloaded has 122000 - an average human has 70000) and are also really agile despite attributes like asymmetrical limbs or backwards joints.
Their attacks also have 2:2 prepare and recovery time as well like other procedural creatures, compared to the 3:3 (or 4:4 for kicks) that non-procedural creatures have, so they also attack quickly. You can also smell them if one is nearby - they either smell like cooked flesh, bug innards, or just "death" (personally I take it to mean they smell like a rotting corpse - reanimated zombies smell like that as well if they're nearby). So while just seeing one would be bad enough, actually fighting one is usually even more terrifying, and it'd probably be an assault on the senses as well given the stench.