One Flew Over The Cuckoo’s Nest
Sure, it's not a classic horror film. I'll give you that.
But it has an evil monster.
At least one scene of torture.
A bunch of crazy people being drugged into a zombie like state.
And the hero has a sharp object inserted into his brain, and is later murdered.
Plus it's set in a freaking mental hospital, which must be a horrible place to be.
So yeah, to me this is a horror movie.