The story
Believed to be Scottish in origin and derives from shanks' nag (shanks-naig 1774), referring to the use of shank to refer to the part of the human leg between the knee and ankle. One theory cites "shank's mare" derived from a horse-drawn lawn mower, manufactured by Shanks & Company Ltd. (founded 1853) which required that the human operator walk behind the device to guide the horse; however, references to the phrase in Scottish literature pre-date the existence of the Shanks lawn mower.
Etymology from Wiktionary, CC BY-SA 4.0.