A dealer in horses: vulgarly and corruptly pronounced HORSE COURSER. The verb TO COSE was used by the Scots, in the sense of bartering or exchanging.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Horse Buss * Horse GodmotherNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes