I got this idea watching a video about Machine for Pigs a few days ago, in which the primary enemies are pigmen. For some reason it got me thining about werwolves, probably because a half-man-half-pig is similar to a half-man-half-wolf. However, one is a person afflicted by a disease that makes him turn under the light of the full moon and invulnerable to anything but silver, while the other is an alchemically warped hybrid of two creatures that doesn’t have any of these special traits.
With a werwolf, you know exactly what you are dealing with. You know what caused it, what triggered it, how the creature behaves, and how to kill it. But while a pigman might also stalk the night an brutally tear its victims to pieces, you don’t know anything about its behavior patterns and how it can be killed. And that’s the key to making horror monsters. Fear is essentially a response to not knowing how to respond to a dangrous situation. When you understand the danger, you can deal with it in a safe way, or at least get yourself out of harms way. You are in control of the situation, so there is no reason to fear.
So when it comes to creating or using horror creatures, it’s vital that the players do not know what they are dealing with. And I think it might be even more effective if the players think they know what they are dealing with and that they are in control of the situation, only to have them realize that the weapons and protective items they brought don’t do anything against the creature. Right now, I really want to make a short adventure in which an unseen creature attacks people during nights of the full moon, leaves behind mangled corpses, and is only seen as a shaggy bipedal shape that jumps in great leaps over roofs and walls. But then it keeps attacking even after the full moon has passed and its entirely unaffected by silver and wolfsbane. Which the players will only realize once they sprung their trap and have the beast cornered.