I'm not really big into horse-riding, but if I were to guess....
it depends on the horse-riding your doing.
If your going slow, the workout is probably minimal. If your riding moderately to fast, the workout is probably significant. You are gaining leg muscles from holding on, and also slight upper body (if your holding with hands)

As for cardio, I would think you must need to be going pretty fast for that. A cardio workout requires rapid muscle contractions.

Test it for yourself. If during horse-riding, if your heart races, and you break a sweat, you are most likely doing cardio!