horn has 10 distinct synonyms spread across 5 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.
a noisemaker (as at parties or games) that makes a loud noise when you blow through it
a noise made by the driver of an automobile to give warning
a high pommel of a Western saddle (usually metal covered with leather)
any hard protuberance from the head of an organism that is similar to or suggestive of a horn
the material (mostly keratin) that covers the horns of ungulates and forms hooves and claws and nails
a device having the shape of a horn
“horns at the ends of a new moon”
a brass musical instrument consisting of a conical tube that is coiled into a spiral and played by means of valves
a device on an automobile for making a warning noise
Synonyms, antonyms and the broader and narrower relations for horn come from Princeton University’s WordNet. Full sources and licences.