Horse riding is a major cardio and muscular workout. The morning of a horse show I take a Tums (nerves) and 3 Advil as soon as I wake up to establish a base for my muscular fatigue. Its like doing squats all day, but if you fuck up your squat position a 1000 lb animal can be like "get the fuck off" and kill you. Although, I will agree that pony rides at the fair are not exercise.  If you ever want to know what it's like, sit on an exercise ball with your feet drawn up an inch or two off the ground. Stand up so that your crotch is off the ball without holding onto anything or touching the ground with your feet. Repeat for about an hour.