collar has 15 distinct synonyms spread across 6 of its 12 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.
(zoology) an encircling band or marking around the neck of any animal
anything worn or placed about the neck
“the thief was forced to wear a heavy wooden collar”
a short ring fastened over a rod or shaft to limit, guide, or secure a machine part
a band of leather or rope that is placed around an animal's neck as a harness or to identify it
necklace that fits tightly around a woman's neck
the act of apprehending (especially apprehending a criminal)
“the policeman on the beat got credit for the collar”
Synonyms, antonyms and the broader and narrower relations for collar come from Princeton University’s WordNet. Full sources and licences.