The way you stated your question makes no sense, you are making it sound as if you are too weak to mount a horse, but the muscles you strengthen when you ride a horse are, if you ride properly, thighs, back, abs, calves, arms and neck. But if you ride horribly and do not position your self right and have terrible posture you stregthen absolutely nothing.
if you sit properly in the saddle, and do a substantial amount of time in each gait, then yes!
(Strengthen your muscles)
Strengthen your abdominal muscles.
To get your legs to strengthen by running and running because this is using your muscles and that helps your legs to strengthen.
Any muscle group in the legs or abdominal areas. Also it wouldn't hurt to strengthen your arms.
The exercise ball will strengthen different muscles depending on how you use it. It's probably most popular and frequently used for an abdominal work out.
When exercise is done the muscles are broken down. The muscles are then repared back stronger.
Strengthen weak muscles and stretch shortened muscles.
Some good ways to strengthen your knees would be to perform exercises which would strengthen the muscles around your knees. Low impact sports would help to build muscles on your knees as well.
http://www.nismat.org/ptcor/lbp/
Squats and poly-metrics.
You can strengthen your muscular system by using it. By lifting weights or running, you are using the muscles, and they are growing and toning. When you don't use your muscles, the are not as strong and you ware weaker as a whole.