existent has 3 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. Its one recorded opposite is nonexistent.
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.
having existence or being or actuality
“an attempt to refine the existent machinery to make it more efficient”
being or occurring in fact or actuality; having verified existence; not illusory
“real objects”
presently existing in fact and not merely potential or possible
“the predicted temperature and the actual temperature were markedly different”
Synonyms, antonyms and the broader and narrower relations for existent come from Princeton University’s WordNet. Full sources and licences.