WordNet (r) 3.0 (2006):

workhorse
    n 1: machine that performs dependably under heavy use; "the IBM
         main frame computers have been the workhorse of the
         business world"
    2: a horse used for plowing and hauling and other heavy labor

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "workhorse":
   beast of burden, bidet, buck, carriage horse, cart horse,
   cavalry horse, draft horse, dray horse, driving horse, drudge, fag,
   fill horse, filler, galley slave, gigster, greasy grind, grind,
   grub, hack, hackney, horse, hunter, jument, lead, leader, mount,
   pack horse, palfrey, peon, plodder, plow horse, pole horse,
   polo pony, post-horse, remount, rider, riding horse, road horse,
   roadster, rouncy, saddle horse, saddler, sawbuck, shaft horse,
   slave, slavey, slogger, stalking-horse, sumpter, sumpter horse,
   swot, thill horse, thiller, toiler, trestle, wheeler, wheelhorse