Green’s Dictionary of Slang

sweater n.3

(Aus./US und.) one who steals a horse, or borrows it without permission.

Bathurst Free Press 25 Apr. 3/2: [A] chase [...] after two ‘sweaters’ (a mild slang term for horse-stealers or borrowers).
[US]Butte Miner (MT) 14 July 4/2: Johnny Murphy, a ‘sweater’ and sure-thing man.