Green’s Dictionary of Slang

satyr n.

[SE satyr, a mythological Greek woodland demon, usu. pictured with the ears and tail of a horse. ‘Men living wild in the Fields, that keep their Holds and Dwellings in the Country and forsaken Places, stealing Horses, Kine, Sheep, and all other sort of Cattle’ (A. Smith, Lives of the Highwaymen, 1714)]

[mid-18C] (UK Und.) a professional horse thief.