I know the answer! Each different breed has a genetical predisposition to develop some muscles or save fat in some places, depending one the function it was selected for. So, the most right way to know if your horse is good shape, is to take a look at the model horses that are choosen to breed. If your horse look like them you are in the right way! And, if it does not belong to any breed, well.. ribs are supposed to be covered, if stalion some fat in the on the neck, if mare a more round shape, if gelding well.... if gelding need to be nice looking without looking rounded.