WordNet records no synonym and no antonym for acorn squash. 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.
squash plant bearing small acorn-shaped fruits having yellow flesh and dark green or yellow rind with longitudinal ridges
small dark green or yellow ribbed squash with yellow to orange flesh
Synonyms, antonyms and the broader and narrower relations for acorn squash come from Princeton University’s WordNet. Full sources and licences.