Many people don't consider horseback riding exercise. They say the horse is the one doing all the work and the person is just riding on top. And while that may be what it looks like, especially with experienced riders who make it look effortless, that is definitely not the case.
Riding a horse isn't really considered a "cardio" workout since you are sitting on top of a horse. However if you ride properly, you are pretty much utilizing all the muscles in you body between controlling the horse, maintaining proper posture and balance, and following through with the different "commands."
There are two kinds of riding styles- English and Western. Since the western riding style was developed according to the needs of cowboys who work with cattle on horseback, this article will be focusing on english riding.
For starters, you must sit properly. You may ask yourself, "sitting on a horse is just sitting right?" Not really. According to Katherine Blocksdorf from horses.about.com, staying on and controlling a horse requires the same sort of body control that many other sports require, like martial arts and downhill skiing. She goes on to say, "you need to learn how to use your body and balance to make your ride more comfortable and safe, and easier for your horse."
Something else to remember is "heels down, toes up" in the stirrups and to always sit up straight. You hold the reins in each hand not too tight but not too loosely either, and not exceeding the width of the horse's neck while keeping your hands a little above the saddle. Now, squeezing the horse's belly with your legs to get it to start walking works the inner thighs. Another squeeze to get it to trot. Not losing your balance during that transition requires core strength, still being mindful of "heels up, toes down," sitting up straight and watching your hands and arms. Now during the trot, you must post. So in addition to all that is mentioned, you must now rise out of the saddle for every other stride of the horse's forelegs, keeping your legs and arms still while your body goes up and down during the horse's stride! Still sound simple? And this is without even getting into how to canter.
So as you can see, even though it's not cardio and you most likely won't lose weight from it, riding a horse properly forces you to use muscles throughout your body that you typically don't use, so even someone who exercises regularly and is physically fit will most likely be sore afterwards!
NOTE: Do not attempt to ride a horse on your own if you never had lessons, or are inexperienced. It is a dangerous activity that requires supervision, especially if you're a beginner.