grass snake has 5 distinct synonyms spread across 3 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
harmless European snake with a bright yellow collar; common in England
any of numerous nonvenomous longitudinally-striped viviparous North American and Central American snakes
either of two North American chiefly insectivorous snakes that are green in color
Synonyms, antonyms and the broader and narrower relations for grass snake come from Princeton University’s WordNet. Full sources and licences.