"It's just mechanical lameness."
Mechanical lameness is an alteration or hitch in a horse's gait that's caused by a physical problem with the joints rather than by pain. A horse with a mechanical lameness is often usable with some limitations, but has to be treated carefully as sometimes the gait unevenness can cause problems in the back or with one of the other limbs.
Such horses, however, would not be up to hard work and a war horse or a modern show horse with a mechanical lameness is generally retired. (In any case, modern show horses have to look sound to be shown). They tend to end up being used to train riders or for light trail riding.
However, the key factor with mechanical lameness is that the horse is not, in fact, in pain and working them is not in any way cruel. It's generally caused by damage to a joint that has caused smaller bones in the joint to fuse, restricting motion.