Don't really have much to add to what's been mentioned so far. In Spanish (well, in México, at least, which is what I'm familiar with) we don't really have a weird literary tradition, or even a horror one per se, despite the abundance of macabre imagery in Mexican tradition. Truth be told, not a lot of people are in tune with said tradition, and some even mock or despise it because foreing culture (or what passes for it) is so much more attractive.
So proper horror authors tend to be few and far between. What we do have, however, is writers, including some big names like Octavio Paz, who ended up writing one or a few stories that could be classified as horror/weird fiction, while it was never their intention to be known as genre writers. (Paz wrote a creepy ltitle tale titled "the blue bouquet", by the way.)
I'll come back to this thread later with, I hope, suggestions of specific stories to look for, plus some more authors I don't know have been translated or not. Gotta look into that.