Since horses have been kept in human captivity they have been bred for desired characteristics. Whereas they used to be selected for meat production and docile temperament, nowadays breeding horses are selected for a wide variety of desirable traits, including many of which are appearance related. For various reasons breeding has led to generally longer legs of the horse. In order to still reach the grass whilst grazing the horse needs to spread its legs. In line with your horse?s functional anatomy it spreads the legs by placing one in front and one underneath the body, rather than to the sides. When in this position, the weight distributions between both front hooves is different: the leg hoof placed in front carries more weight at the back whereas the hoof placed underneath the body carries more weight at its front.
A study  concerning twenty-four Warmblood foals showed that about 50% of the foals demonstrated a clear preference to put systematically the same leg in front of the other. Consequently both fore hooves adapted their shape in response to the different weight distributions between both fore feet; the hoof placed in front became flatter and wider, the hoof placed backwards became more upright. It was further recorded that after uneven feet had formed, this new shape also led to uneven loading patterns in the hoof when horses stood up straight with the front legs next to each other. In line with the previous paragraph, foals with relatively long legs and small heads were far more likely to develop a ?preferred front foot? than foals that could reach the ground with their mouths more easily.
The technical term for horses to have a preferred side is called laterality. Laterality shows, as explained, in the position of the legs when grazing, but also during movement in for example dressage exercises. In grazing position the leg placed in front is called the leading leg, the leg placed under the body is called the trailing leg. In canter the leg first touching the ground is on the leading side. If a horse has laterality, the leading leg in grazing position is almost always also the preferred leading leg in canter. For example, if a horse prefers to graze with its left leg to the front it is most likely to prefer cantering clockwise over counter clockwise. Laterality is something to take into account when purchasing and training a horse. If you want a race horse of which you know it will need to run predominantly in a certain orientation (clockwise or counter clockwise) I would recommend to keep the horse?s laterality in mind. Also, when training your horse to do exercises, allow time for it to learn them on both sides, even if you consider the exercise easy yourself. Imagine tooth brushing with your opposite hand; the action itself is not difficult, but doing it with your non-preferable side is. The only way to learn is with training and time whereas force, anger and frustration will rather have an adverse effect.