As appealing as a horse's back looks to sit on, it wasn't designed to carry people--or anything else for that matter. If somebody built a table the same way a horse is put together chances are you'd think they needed to go back to carpenters' school. As a piece of furniture, it'd be too tippy. Doubt me? Just give one of your Breyer horses a little nudge and see what happens (yes, I know you still have them).
On the other hand, when the real deal is moving he's a wonder of grace and power. Until a human climbs on.
Putting a rider on top alters the balance of the horse. If you've ever been the first one to ride a young horse, you know all about that wobbly, unsteady feel and how many months it takes a horse to adapt. The more skilled the rider is in maintaining her own independent balance, the easier it is for the horse and the faster he regains his natural elegance. A good deal of a horse's first training has to do with helping him rediscover his natural way of going while being ridden. This is why it's so important that a young horse is brought along by an experienced, skilled rider. Put a beginning rider on a green horse and they'll both be scared--and develop some really bad habits, all in the name of survival. In the same way a skilled rider helps a young horse, a well seasoned horse will help a beginning rider.
As we acquire more skills and confidence, we sometimes forget that balance is still an issue. Even at the highest levels, the basic lessons of maintaining accord between our own center of gravity and the horse's is vital if we are to achieve that effortless, relaxed performance we chase like the Holy Grail.
So what is the message I've taken away from this horse-taught lesson in kinesiology? The value of patiently maintaining one's equilibrium while someone else is trying to gain theirs. Sometimes the very best help you can offer is to pay close attention to your own balance while the one close to you regains theirs.
Eventually, you will dance together.