What observable factors or qualities can you use to help determine a horse’s trainability?

I will offer my opinion here, but this ends up being rather a subjective question, as there are several variables – not the least of which is “who’s doing the training?”
For a novice learning to train horses, it is really more about “tractability” than trainability, right?? This to say:? just because a certain horse lacks the temperament to be trained by an amateur at a horsemanship clinic does not mean that the horse is not trainable – only that they may be excitable, sensitive, and/or have issues attributable to previous mishandling (or lack of handling).? Observably, such horses will tend appear more relaxed.? They will seem to enjoy attention – or at least not mind it; lack any tendency to resort to defending themselves when being handled (bullying, kicking, biting, general restiveness).? Naturally, they should be sound and free of pain with reasonably good vision.? Ignore completely myths associated with colors, markings, etc.
For an experienced trainer – at least in this experienced trainer’s experience – there are really no specific factors that accurately predict a horse trainability in advance of actually training him.
The horses I have trained to the highest levels the quickest have often been the most difficult to get going initially.? The same intelligence that enables them to really focus at a high school dressage level can make them seem stubborn early on.? Excessive energy and restiveness – once shaped – can translate into easy impulsion later, allowing movement to become its own reward.
Broadly, I follow the training approaches used by, essentially, William Cavendish, 1st Duke of Newcastle, and Francois Baucher.? Baucher was renowned for his ability to rapidly train supposedly untrainable horses – including horses well known for unseating their riders with prejudice.? His experience was that by conducting essential lessons in the right order, virtually any horse may be made acceptable for outdoor performance, and most for high school.
This has been my experience as well.? In fact, I had originally outsourced the basic training (backing, three gaits) of the horse in the following clip due to my travel schedule at the time.? The trainer – a respected, experienced professional – sent him back to me unfinished due to his tendency to unseat his rider if asked to canter – he had become a health hazard, and, under a different owner, likely would have been branded untrainable from there.
As you can see, he turned out to be quite trainable after restarting, offering his piaffe within a couple of months; and completing his first fox hunt soon after.? This could not have been predicted from observable factors early on, which would have suggested the contrary.? Good fit for someone just learning?? No.? Rather a deathtrap.? But with the right approach and a little patience, a perfectly balanced riding instrument who is as happy to carry novices on the trail as he is to give butterflies to connoisseurs in the rectangle.