what the above poster said is likely right. May not necessarily be in the hocks, but could be in the knees, withers, shoulders, back, lumbar, etc. one way or another it's due to an unevenness in the gait or stride, and/or an unsoundness/unevenness in the muscling of the horse. It could be chiropractic, arthritic, really anything. Without seeing the horse move, it's hard to say WHY, other than if you're looking to purchase a hunter for your daughter where movement counts, or even a jumper where speed an balance are super important, I wouldn't take on the horse without a significant reduction from asking price and willingness to invest time and money into finding out, addressing, and fixing the issue at it's core.