Introduction:
In the given sentence, the correct preposition to use with the verb "ride" is "on." The sentence should be written as "I ride on a horse." This is because "on" is the preposition commonly used when referring to riding or mounting a horse.

Explanation:
- The preposition "on" is used to indicate a position or location on top of something. When it comes to riding a horse, we usually sit or position ourselves on top of the horse's back.
- When we say "I ride on a horse," it implies that we are sitting or mounted on the horse's back while engaging in horse riding.
- The preposition "in" is used to indicate being inside something or within boundaries. It is not suitable in this context because we don't ride inside a horse.
- The preposition "down" is used to indicate movement towards a lower position or level. It is not appropriate in this context as it doesn't convey the action of riding on a horse.
- The preposition "at" is used to indicate a specific location or point. It doesn't convey the action of riding or being on top of something.

Summary:
In summary, the correct preposition to use in the sentence "I ride ____ a horse" is "on." This preposition accurately conveys the action of riding or mounting a horse, indicating that the person is sitting or positioned on top of the horse's back while engaging in horse riding.