This seems to be a much debated topic. There is an even divide in response; some people say yes, and some people say no. They think that it hurts the horse, or that it is not natural because the rider’s weight is not distributed as evenly as it is in a saddle. I say, it depends; on the horse and on the rider. Is your horse bony-backed? Are you an especially thin person? If so, you should not ride your horse bareback; horses have extremely sensitive skin and can feel things very harshly. You would dig into his back muscles and cause his back to be tender so that you could not ride him for several days. Riding bones on bones does not mix. A large rider on a narrow or bony horse is also not a good idea; it will put too much pressure on one area of the horse’s back; thus causing the same problem: a sore horse. Fattening the horse up will probably not work, some horses, like some people, are just built very slender and there is nothing to do about it.
Another factor is your seat. How do you sit on the horse? Is your weight always in one area, or are you bouncing around? Do you move with the horse, or clamp your legs against his sides in an effort to stay on and stay still? If you sit stiffly on the horse, with your weight all in one area, it can be very uncomfortable for your horse and he will not ride well. His gaits will be stiff and make your ride difficult. Do not clamp your legs on the horse either, he may take this as a response to move faster and while you try to stop him, he will get confused thinking that he is being told to go faster. This all factors into whether you should ride bareback or not. I was raised riding bareback first, most people are taught in the saddle first, and so while I find it easy, comfortable, and relaxing to ride bareback, you may not.
Develop an independent seat. Feel as comfortable riding bareback as you do with a saddle, and always, no matter what, start out at the walk until you feel secure and then you may move up to faster gaits. Do not ride at faster gaits until you are certain that you are able to; if you fall off, you may lose your confidence all together. If you decide to trot, start out at a slow trot, and if you feel good about it, and comfortable doing it, go faster. The most important thing to remember while riding bareback is: move with the horse. If you had to carry something heavy around on your back, would you rather it stay stiff and hard on your back, or move pliably with you as if it were part of you? Of course you would want it to move with you, so do the same with your horse. Make his load easier by not being a burden.