Among the movies, what percentage are horror?
horror is a name of film category; calculation = DIVIDE(COUNT('horror'), COUNT(category_id)) * 100
SELECT CAST(SUM(IIF(T2.name = 'horror', 1, 0)) AS REAL) * 100 / COUNT(T2.category_id) FROM film_category AS T1 INNER JOIN category AS T2 ON T1.category_id = T2.category_id