The way a horse is built is known as its conformation, and when it comes to dressage, there are two practical reasons for looking at it: because the shape will make the work easier and because it will help the horse to remain sound as it carries out that work for many years.
You don’t need a veterinary degree in order to get a good overall impression; just step back and look at the horse from the side in a straight-on view. The most useful thing to notice is an uphill balance, that is, the withers at the same level as or higher than the croup and the neck emerging from the shoulder high rather than low. Since dressage involves shifting the weight to the hindquarters, a horse that has a slight uphill balance begins each stride with the advantage. Although a horse that is built downhill can still carry out good work, it is always having to struggle with its balance, and the more difficult the work becomes, the greater the cost of that struggle.
Now consider the areas that carry the main weight. The rear section and hind legs are the most important, as collection occurs in that area. Most purchasers are drawn in by the showy front legs and fail to look at the hind end, where the horse’s future is ultimately determined. The picture is completed by a strong, well-connected back and loins, correct legs, and good feet. It is normal to find some minor flaws and they are seldom serious. The faults you should be looking for are those that place excessive strain on a joint, since it is precisely these that tend to exhaust a horse prematurely.
Movement is the other half; when judging a dressage horse you have to consider the three gaits, and there is one piece of advice which goes against the natural instinct of most buyers;t hat is, the walk and the canter are more important than the trot. The trot is the gait that is most often shown in videos, featuring a lot of suspension and knee action, and it is also the one that training in gait work can improve the most. It is much more difficult to correct the walk and the canter. A clean, marching walk with a good amount of overstep is worth more than it seems, and a poor walk is almost impossible to fix. At the higher levels, success is based on an uphill, balanced canter with an active hind leg. When you are observing a horse’s movement, make it a point to ignore the front legs for a full circle and instead look at the hind legs and the back. If the movement then appears satisfactory, it is satisfactory.
Above all, one should be a bit suspicious when observing a very impressive young trot. A massive, floating trot at five years old may mask a hind leg that is unable to collect, and this fault only becomes apparent later on when the horse has to do the actual work. The issue is not at all about how impressive the horse looks as it goes by a camera; it is whether or not the horse can sit and carry, something that is harder to see but more important.