True horsemanship isn't about training one horse after another the same way, seeing what sticks and discarding the rest. That is a short sighted wasteful way of thinking. To find every horses' talents and train toward that is the art. Feeling her out, listening, seeing movement are all elements to find her place. It is easy to see these elements in the young untouched horse and is a simpler process to start training. It is much more difficult in a "second hand" or rescue horse. The questions asked should be what was the previous training? Was it enjoyed by the horse or just tolerated? What positive or negative experiences have shaped their lives? Which directions should we go in? Many questions you need to ask this horse. I do not enter training with a horse with expectations. Starting out I will use much of the same technique but I am looking for small responses. Those little indications from the tension, ears, attitude, type of stride, ease of learning etc. This will tell me where to go. Sometimes the challenge can be to loosen the tension enough to see the horses' needs and personality. Many tense horses will go through the motions just to please their handler but will never show the real personality and ability of that horse; with some horses it is very difficult to get through this tension barrier and can be paralyzing to that horse.
Age can be factors in training as well. The young horse can be developed through growing years and groomed into an excellent performer. But the second hand aged horse over the age of 8 needs a more specialized approach. A green broken 15 year old horse who has a good walk, fast canter and choppy trot needs something special. Narrowing down possibilities due to the fact that she doesn't have time to develop her trot for two years (a trot she's been comfortable with for 15 years...not happening) so I need to find her a place with what she has now. Good walk with reluctance to break into her choppy trot makes for a great hobby trail horse. Her excellent stop and fast canter are great for barrel racing. At those times she would just need to learn movements while building muscle for those things without needing to take years relearning her own gaits.
Using this process is also good for finding injury. Horses are prey animals, so they hide injury and pain to prevent predators from singling them out as weak. So when you see obvious pain and lameness in a horse it is then beyond what he can reasonably handle and more serious. The earlier you can spot lameness and injury the faster and more likely he will recover completely.
Being able to create a specialized learning environment for every horse within a couple hours session is a true art needing apprenticeship to truly learn. Much can be learned in the classroom and theory techniques can be taught in a group. But the feel, balance and listening to the horse must be learned with someone who has been doing it for years. I do not claim to be a master; I have not yet finished a horse to relative perfection, But I have studied with them. I have completed my apprenticeships and have moved on to making my own experiences; a journeyman.