Natural asymmetry and locomotion

The hoof has to fulfill two important functions. It needs to carry the weight of the horse, and it needs to balance the horse during locomotion, when at some points in time, the horses' weight is carried by a single hoof.

The illustrations below are referenced from this very informative website.

The static load is distributes equally on both front legs:

The dynamic load however can be concentrated on either one of the two front legs:

In order to balance the weight of the front half of the horse on a single hoof, the horse loads the lateral side 10-20% more:

I have found, that the horse performs this "balancing act" by moving the leg slightly towards the mid-line.

My horse Gus provides a good example for this. He has rather straight legs (very slightly base narrow) and I have not seen any horse with more healthy and regular hooves than his, yet he places his front feet in such a way that they load the lateral side more, by placing them towards the midline. This specific way of walking is more pronounced in soft ground, most likely as there is more need for balancing the weight on soft ground. But it also can be seen on harder ground.  On a level surface, like a tarmac road, Gus lands heel first or flat and this placement of the legs towards the midline is not very obvious, but to some degree it can be seen even on a hard flat surface.

It is interesting to note that the same conclusion has been drawn by the author of this website:
http://otherideas.typepad.com/christine_sander_hufbalan/

The article is in German, but the first picture shows this specific way of advancing the legs towards the midline.

Taken together these observations suggest (as has been done here) that the lateral side of the horse's hoof is wider and flatter because it is the "balancing side" of the hoof, and the medial side is primarily involved for the static load bearing of the leg.

It may also be worth noting that as the weight of the horse increases, so is the need to balance the horse's weight during locomotion, when temporarily only one leg carries the weight. The heavier the horse, the more balancing on the lateral hoof side is required.