There is no absolute rule on how much weight a horse can carry. It is highly dependent on factors like breed, conformation, conditioning, and discipline. Certain breeds such as the Quarter Horse, Arabian, Icelandic Pony, and draft breeds are more suitable for carrying heavier loads. Horses that are used for light pleasure riding can carry more than a horse being asked to ride endurance.
A generalized estimate for the total weight (rider, tack, and gear) that your horse can carry is 20% of its body weight. For example, a 1000lb horse should be able to safely carry 200 pounds. For lighter pleasure riding, this value can be slightly higher, closer to 30% of the horse’s weight.