25% slightly more or less is the general rule of thumb. However, it does go a bit deeper than the general rule of thumb.... The age and condition of the horse is paramount here! A young horse has not yet grown enough (if under the age of five) to have had all the bones grow and fuse as they should...(the biggest reason that Race Horses BREAK DOWN! Their legs and backs aren't ready for such grueling use) A horse in poor condition health wise should not be expected to carry much weight, even if moderate riding is recommended by a Vet as part of the recovery process. If it is a mature horse, what is the current condition of the feet and legs and back? If the horse has had an injury or Ringbone, thrush, poorly cared for feet, any swelling or heat to the legs or joints, then you need to ere on the side of caution! How does the saddle fit? If it's too big (wide) or too small (narrow) you are asking for problems from the get-go..even without adding additional weight. Does the horse need shoes to be comfortable? Have the hooves been properly trimmed or worn down correctly? All of these are considerations to keep in mind before you or anyone rides any horse.