a The word 'horror' doen't start with a vowel sound.
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a horror film
The tendency nowadays is to use 'a' before a word that begins with 'h' unless the 'h' is silent (an honor, an hour, an honest man).
Even the old chestnut 'an historical ...' was surpassed by 'a historical ...' in frequency of usage as long ago as 1940 according to the Google Books corpus.
CJ
(x-post)