Look at Cujo. The book, and subsequent movie, are visceral and terrifying–and yet, they are about a rabid dog, not some demonic creature. Any dog can be bitten by a rabid animal and go on such a rampage. The story terrifies because it could happen to any household in any neighborhood.
If you are setting out to write horror, don’t think only of the monsters. Think of the mundane items in your world and see if there is something you can turn into the catalyst of your story. Perhaps the water fountain in the square suddenly starts poisoning people. Why? Is it a human intervention, or is there something more devious at work? Perhaps a vengeful water spirit?
Maybe a pet songbird suddenly starts singing words–words that prophesize murder. (Wait…I think I want that one…)
The point is, horror doesn’t have to be full of ghosts or demons or vampires to bring a shiver to the spine. And, sometimes, it is better when it isn’t.