To whet your appetite and keep your writing fingers limber, take a good look at the photo, or let a prompt settle in your mind. Set a timer for a reasonable amount of time, be that five, fifteen, or thirty minutes. Then start writing.
Tell the story of a man on an ordinary trail ride whose horse suddenly grows wings or a unicorn horn.
If you could ride a horse anywhere, where would you ride? For example, would you want to ride a Connemara pony in Ireland? Or take a horse trek through the Patagonian foothills of the Andes?
Write an article or blog post telling people about vacations that include horseback riding.