From the point of view of the bettor, they only see the superficial things in horse racing. Things like the form of the horse, the previous wins, the speed of each animal - these are taken into account.
Form the point of view of the horse owner, the concerns are greater. Horse owners have to consider the usefulness of the horse in the races and years to come. From their view, the horse can only work for some time before age and wear and tear factors affect it.
Most horses are brought up in traditional horse training techniques that require horses as young as 2 years old to begin training. If a horse goes beyond the age of 4 years old without any type of training, then the horse may be considered too "old" for the sport.
Most horses are thoroughbreds and these type have come from a strain or breeding of Arabian horses. Careful breeding has to be done so that the strength and character of the horse is maintained. Any mis-breeding or introduction of a different breed in to the process usually results in a mixed and unstable attitude of the horse. This plus the fact that the physical result would be inferior.
There are some horses that have a varied response to being handled. Some horses take to the training regiment very quickly while others need a longer time to assimilate the information needed. Usually horses are a bright and intelligent lot and it would really take a stubborn horse not to follow training.
As a rule in training, there can be a combination of "rocks and salt". This is a term sometimes used by old fashioned trainers that advocate the punishment and reward system of training. Reward the horse for good deeds, punish for not following instructions.
Although some horses may make it pas the training stage, it is the performance or progression that makes the owner decide if the horse was worth the gamble. Progression means a noticeable change in the performance of the horse in the races and trial runs. Only when the horse performs for real in a race can the horse be truly worth its weight.
Horse racing alone is very challenging but being a horse owner and also a horse raiser can be very hard at times for it has been compared to being a parent only difference is the horse is worth quite a lot.
One does not really have to know what goes into making a "winning" horse, as long as one enjoys the game, that should be the real decision.