http://www.oricomtech.com/projects/leg-anat.htm

Stability. Back to the horse, what is of prime interest here is that, like the shrew, the upper leg segments [4-5, 12-13] of the horse both angle inwards, centering the weight of the torso and giving the frame front-back stability. One might surmise that a similar arrangement would prove useful in the design of a robot quadraped, and in fact, this biological lesson was not lost on the designers of the Sony Aiborobotic dog.

What is clear is that the mirror image geometry of the legs keeps them symmetrically-positioned with respect to the COG of the torso at all points in the stride - and this helps keep the horse on an even kilter as it runs. Compare running dog with running horse and panther - front-back legs movement are similarly symmetrical in all cases.

Standing Up. The mirror image geometry of the legs certainly helps keep the COG of the horse's torso centered during the stride, but the long lower leg segments represent a difficulty when the horse (and similar animals) try to rise from a kneeling or sitting position. That the animal's major muscle groups reside near the body, and the animal essentially has to "lever" itself up, certainly contributes to the problem. Animals with shortened forelimbs, like the dog and cat, have a much easier time of it. This is just a passing observation that may have some bearing on the design of robot legs.