There is a very good stables in Salou,who have many horses to ride. However, in September, I seriously doubt there is a beach near enough,that is free from people to allow you to ride along. Secondly, I doubt that the stable proprietors (Hipica Westfork)would be prepared to take you and a horse far enough away to find a beach like that to ride along. No harm in googling 'horse riding on beach in Catalunya'...you might find something!
here, try this site:
http://www.panorama-trails.com/spain/horse-riding/Catalonia-beach-ride-weekend.html
Just copy the link to your browser address bar.