The legs could be said to be the most important part of the horse, for if a horse has weakness or bad conformation in his legs, his athletic ability is going to be seriously compromised, no matter what you plan to do with him. From a general standpoint, the legs should appear straight, muscular and sturdy, and capable of carrying the horse.
On to Specifics
Looking at the horse head-on, the front legs should be neither too far apart nor too close together as they join the chest. Too far apart will result in a rolling gait, that may make you sea-sick to ride. Too close together, they will predispose the horse to injuries as one foreleg knocks against the other as he moves. This interference may be alleviated somewhat with special hoof trimming and shoeing, but is better avoided in the first place. The legs should appear straight and not twisted in or out.
The forearm should be long and muscular and the knee should be large and flat, not round and puffy. The tendons have to pass through the knee to the lower leg and that is why large, flat knees are desireable, to allow the maximum movement.
The cannon-bone, the bone in the lower leg, should not be too long. Looking at the horse from the side, the knee should appear low in the leg. If it appears high, it means that the cannon bone is longer than ideal, and prone to weakness.
The way the lower leg joins at the knee is important to the functionality of the leg too. In some horses you will see that the lower leg appears tied in below the knee , where the leg is narrower directly underneath the knee than it is further down. This can restrict the movement of the tendons. In some cases the horse may be over at the knee where the lower leg appears set back in comparison with the upper leg. The opposite of this is back at the knee where the knee and upper leg appear set back in comparison with the lower leg.
As the horse's ability to carry weight depends on the inter-functioning of all these parts, it is important that they line up correctly, in order to be most efficient.
The pastern is the horse's shock absorber, at times carrying the horse's entire weight, plus that of the rider. The ideal pastern is neither too long nor too short, too sloped or too upright. Overly long sloping pasterns will place a strain on the suspensory ligament and the tendons which run down the back of the leg. Pasterns which are too upright do not perform sufficiently well to overcome the concussive effects of movement and so the leg may suffer with soundness problems because of it.
Equine Anatomy - The horse's hoof
You know the saying "No hoof...no horse", well - never a truer word was spoken. The ideal hoof is well-matched with it's partner in size and shape (hind feet will be bigger and the footprint more oval than the front feet) The horn should be strong and flexible, not weak and shelly or dry and crumbly. (I plan on doing some extensive research and bringing you the latest information and help on how to keep your horse's hooves in tip-top condition). Problems with the hoof walls will mean that the horse will have difficulty holding on shoes (a problem I am all too familiar with). Different breeds of horse tend to have different sized feet. The draught breeds have huge soup-plates of feet whereas the Arabians have small strong-horned feet. When trimming and shoeing the horse it is necessary to maintain the natural size of the foot, not to trim it down to fit the shoe, as this will cause problems with soundness, by compromising the natural shock-absorbing qualities of the hoof.
The feet should point straight forward and the horse be neither pigeon-toed, where the toes point in toward each other, nor toed-out , where the horse appears duck-footed with his toes splayed out.
The feet should have fleshy, well-sprung frogs and should not be boxy and upright, like those of a donkey. Lateral ridges around the hoof wall can indicate previous laminitis or, at the very least, changes in growth rate due to illness or changes in feed etc.
Equine Anatomy - The hind legs
The hocks are one of the very important features of the hind leg. This acts as an additional shock-absorber. Like the knee, it is preferable for the hock to be set low, thus lessening the strain on the hind cannon. How the hock lines up underneath the hind quarters of the horse will make a difference to its action. Ideally, in the riding horse, the point of the hock should line up underneath the point of the buttocks and a straight line be carried down the back of the hind cannon. However, hocks that fall in front of this line are often seen in jumping horses, as they give additional thrust. Horses with hocks that fall behind this line will be lacking in propulsion, making jumping more difficult for them and well as making it difficult for them to perform the more advanced dressage movements.
Looking at the horse from the back, the hind legs should be straight and neither cow-hocked, where the hocks turn in (accompanied by turned out toes) or bow-hocked , where the hocks turn out. From the side, the horse should not be sickle-hocked, where the front edge of the leg appears overly curved or bent. All of these conformations can cause soundness problems by compromising the way the shock absorbing qualities of the hock are supposed to work.
Equine Anatomy and Conformation - The whole picture
If all of the above points have been taken into consideration, you should be looking at a horse that is well-balanced, nicely proportioned and physically capable to doing what you want him to do. As I said at the beginning of the Part One of this article, there is no such thing as the perfect horse, but being knowledgeable will help you find the best horse for your needs.