WordNet records no synonym and no antonym for workhorse. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
machine that performs dependably under heavy use
“the IBM main frame computers have been the workhorse of the business world”
a horse used for plowing and hauling and other heavy labor
Synonyms, antonyms and the broader and narrower relations for workhorse come from Princeton University’s WordNet. Full sources and licences.