In my story, there's a thing very similar to the shadow, the presence: It's always there, hunting you, appearing in flesh shape (too lazy to make extra models...) and it comes from a world where the worse nightmares are possible (the world of madness). This presence is your main enemy, as, even going to different worlds (up to 4) it still tries to hunt you, and the player doesn't know when or how it will do it. It may invoke a shadowlurker (kind of a ghost) or the already mentioned flesh. In a horror game, you must keep the player tense and peaceful at the same times, giving fear and peace injections regularly, so each one of them has effect.
Another thing is that I like to play with people's minds, as FG did with me: Most of the horror comes from a person, and, more accurately, from the person imagination. People usually tend to dramatize stuff, because when we were in our prehistoric era, our fear sense made us imaginate terrible things in the night when we heard a noise, (it could be a predator). That same and terrible fear saved us. The fear of the unknown, of what we don't know.