Write SQL query to solve given problem: What is the average number of horror movies among all movies genre?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT CAST(COUNT(CASE WHEN T3.genre_name = 'Horror' THEN T1.movie_id ELSE NULL END) AS REAL) / COUNT(T1.movie_id) FROM movie AS T1 INNER JOIN movie_genres AS T2 ON T1.movie_id = T2.movie_id INNER JOIN genre AS T3 ON T2.genre_id = T3.genre_id