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)]

(UK Und.) a professional horse thief.

[UK]C. Johnson Hist. of Highwaymen &c. 186: There is in Ireland, a Sort of Men, whom we may properly enough call Satyrs, from their living in the Woods [...] These People never came to any Towns, but continue in their private Holds, stealing Horses, Kine, Sheep, and all sorts of Cattle.