Windpuffs are considered to be a type of blemish found in the lower legs of aged athletic horses. They are a form of chronic joint distention and appear as soft, fluid-filled swellings near the back of the fetlock that have been present for over a month. They are primarily caused by repeated bouts of synovitis, degenerative joint disease, bone or cartilage defects within the joint (bone chip, osteochondrosis lesion), or stretching of the joint capsule.
As long as there is no heat, the swelling doesn't increase with exercise, and the horse is not lame, then 'windpuffs' are considered a cosmetic blemish. However, each case of chronic joint distention does have a underlying cause which may affect the future athletic career of the horse.