The worst possible location to carry weight is behind the saddle, over your horse's kidneys. Large saddlebags and saddle packs designed to carry overnight camping gear tempt you to fill them with unneeded items, to the possible detriment of your horse. Keep your saddlebags small and light. If you must carry heavier items, such as water bottles or backpacking stoves, select pommel or horn packs that fit on the front of the saddle.
Learn why you should pack your saddle light for you trail ride.