If you write horror flash fiction/short stories and are interested in submitting your work to possibly be published, here are a few sites to look into.
Lamplight: A quarterly magazine of dark fiction. We are a literary magazine of dark fiction, both short stories and flash fiction. We want your best. But then, doesn’t everyone? No specific sub-genres or themes, just good stories. http://lamplightmagazine.com/submissions/
Dark Moon Digest: A quarterly horror magazine, is dedicated to provide quality fiction (from new and established writers) to fans of the horror genre. https://darkmoondigest.submittable.com/submit/21847
HelloHorror: HelloHorror accepts works of fiction, non-fiction, micro-fiction, poetry, photography, visual art and film. All entries must be in English. Translations are acceptable and should be accompanied by a copy of the original text. While we entertain most works associated with the genre, please keep in mind that our goal is to offer something new within the genre with a focus on the psychological aspects of horror. http://hellohorror.com/submit.htm
Pseudopod: Pseudopod is a genre magazine in audio form. We’re looking for horror: dark, weird fiction. We run the spectrum from grim realism or crime drama, to magic-realism, to blatantly supernatural dark fantasy. We publish highly literary stories reminiscent of Poe or Lovecraft as well as vulgar shock-value pulp fiction. We don’t split hairs about genre definitions, and we do not observe any taboos about what kind of content can appear in our stories. Originality demands that you’re better off avoiding vampires, zombies, and other recognizable horror tropes unless you have put a very unique spin on them. What matters most is that the stories are dark and compelling. http://pseudopod.org/guidelines/
Flashes in the Dark: Flashes In The Dark is looking for well-written short, horror fiction under 1,000 words. However, we are looking for real stories…not vignettes, jokes, or horrific descriptions. All stories should have a beginning, middle, and end. Just because they’re short, short stories, doesn’t mean that all the regular elements of a story shouldn’t be present: characterization, setting, conflict theme and a plot. That’s the joy – and the agony – of flash fiction, cramming all those elements into 1,000 words or less. http://flashesinthedark.com/submission-guidelines/