Answer

By Your Horse

Vet Gil Riley explains:

Stringhalt is an abnormal gait of one or both of the hindlegs that causes an exaggerated flexion of the hock joint, resulting in a rapid elevation of the hind limb. The abnormality might be subtle, such as minor upward jerking of the hindleg, or so severe that the canon bone and fetlock are pulled violently against the horse’s belly. Signs could be exaggerated in nervous or agitated horses, or when they are backed.
It is usually only present at walk. It is unusual for the condition to worsen, so my advice would be that, if you are happy with the horse in every other way, I would not let it put me off having him.
Where stringhalt is very pronounced, a surgical procedure can be performed where one or more of the extensor tendons running down the front of the affected limb is cut. The success of this procedure is variable.