WordNet records no synonym and no antonym for fur seal. 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.
an eared seal of the northern Pacific
eared seal of the southern hemisphere; the thick soft underfur is the source of sealskin
“the male fur seal is much larger than the female”
Synonyms, antonyms and the broader and narrower relations for fur seal come from Princeton University’s WordNet. Full sources and licences.