A horse used for draught, ploughing, farm work, etc., rather than for riding or racing.

‘A familiar expression for a common or labouring horse’ (Nares). Obsolete [It is doubtful whether the sense is ‘cut-tail horse’ or ‘gelding’.]

a horse used for ploughing, hauling, or other farm work; a horse of a kind bred for such work.

(a) chiefly Australian and U.S., a horse used for work at night; (b) = nightmare, n. (a punning alteration).