You evaluate a gaited horse the way you do any other horse:
[B]TEMPERMENT
CONFORMATION
WAY OF GOING[/B]
Temperament is an easy one. Any decent horseman can call temperament in a very few minutes of examination.
Conformation is also fairly universal. There are a few differences with gaited horses but they are more “slight variations on a theme” than serious differences. For example, the rear leg of a lateral horse might be slightly longer than you’d find in a trotter. It ought not to be crooked (“sickle hocked”) but this slightly longer leg will allow more “over stride” and will produce a smoother ride. Of course there can be too much of a good thing; this is where an eye experienced in looking at gaited horse conformation comes in.
To look at gait take an 8 x 11 piece of paper and turn in sideways. Draw a line from left to right (or right to left ;)).
Mark the right end “D” (for “diagonal,” meaning trot, the two beat diagonal gait with a moment of suspension) and the left end “L” (for “lateral,” with the end being the true pace). In the center mark it “CG” (for “center gait,” the synchronous, four beat gait).
Now you can take any gaited horse, look at its way of going, and put it on your line. Each breed has names for each of these points (“rack,” “stepping pace,” “paso fino,” “running walk,” “foxtrot,” “marcha batida,” “pasa trote,” etc.). These names can very often confuse a newbie who’s not familiar with the terminology.
The “line method” solves that problem. If you see a horse that is doing a synchronous, four beat gait then you know it’s centered and you don’t have to try and figure out whether it’s a “rack,” a “running walk,” a “flat walk,” or a “marcha de centro.” If you see at horse with a slightly lateral gait you don’t have to worry about a name, all you have to do is say “that looks like an L2” (a lateral gait, just slightly to the “pacy” side). Ditto for gaits to the right of the center march.
The vast majority of North American gaited horses are lateral movers. I’m less familiar with the Central American horses (Paso Finos and Peruvian Pasos). Or Marchadors originate in Brazil and have the broadest range of gait (as they can be lateral, diagonal, or centered and still pass inspection).
The type of gait performed flows from the conformation and the brain of the horse. The brain sends the movement signals and the body responds. In the gaited horse strength, conditioning, and fitness are critical to gait quality. The lateral gaits take more energy than the diagonal gaits. They also require the horse travel with a hollow back. The higher the “L” number the more hollow the back will be. This is not a problem if the rider understands this and mixes the gaits they go (including the canter, and by this I mean a real three beat canter not a “rocking chair” canter).
To get an idea of just how the lateral gait works perform this exercise, first described by Dr. Deb Bennett. Get down on all fours on a carpeted floor (for comfort) and “walk” using the normal equine foot fall sequence. Now “trot” moving diagonal pairs. Don’t try for a “moment of suspension.” Get a feel for the movement. Then “pace” using lateral pairs. Again, try and get a feeling for the movement. If you are doing it correctly then you will find the “pace” exercise is considerably more challenging and will require more energy. A correctly performed center gait (running walk or equivalent) is a relatively low energy gait. The trot is next, followed by the pace.
Since a human is not a horse the above exercise is an approximation. But it it’s useful as a demonstration of the energy budget in types of movement.
Dr. Bennett also points out that every horse can perform just about every gait. By selective breeding we humans have “concentrated” gait tendencies, but we don’t, necessarily, eliminate all others in the process. So it’s not uncommon to find that a Walker will gait beautifully under saddle but trot like warmblood in the field. This is not a problem (although the prospect horrifies certain Walker people). Some breeds produce horses with an “extra gear” routinely. A lot of the PRE horses will do a “suspension less trot.” Generally it’s not a problem and it’s dealt with in the training process. Some PRE enthusiast actually like it. It even will show up from time to time in certain QH lines. Folks sometimes forget that when Capt. King began his breeding program he used American stallions on the very common Spanish mares of South Texas. There’s more Spanish blood behind some QH lines that people remember and this way of going shows up from time to time. Old time cowboys loved it as it was comfortable to ride; modern QH breeders are horrified by it and will “cull” horses that produce it.
I think my “novella” has gone on long enough. 
Suffice it to say that the selection criteria for a gaited horse will not vary significantly from those for a trotter. It’s still temperament, conformation, and way of going. On that last one some specialized knowledge may be helpful, but probably less than some “gaited horse professionals” would demand.
G.
