I don't think horror is inherently less objectionable than porn. Pictures of people having sex isn't going to traumatize any mentally healthy child, but graphic horror might. In extreme causes, I'd say that if you can stomach certain graphic horror, that's a sign that you aren't mentally healthy!
That said, there are obviously different types of horror (and different types of porn for that matter), including child-appropriate horror, so it depends very much on the specifics of the work in question.