manatee vs sea cow

manatee and sea cow both are nouns.

Word	Noun	Adjective	Verb	Adverb
manatee	Yes	No	No	No
sea cow	Yes	No	No	No

As nouns, sea cow is a hypernym of manatee; that is, sea cow is a word with a broader meaning than manatee:

manatee: sirenian mammal of tropical coastal waters of America; the flat tail is rounded
sea cow: any of two families of large herbivorous aquatic mammals with paddle-shaped tails and flipper-like forelimbs and no hind limbs

Other hypernyms of manatee include sirenian, sirenian mammal.

manatee (noun)	sea cow (noun)
sirenian mammal of tropical coastal waters of America; the flat tail is rounded	any of two families of large herbivorous aquatic mammals with paddle-shaped tails and flipper-like forelimbs and no hind limbs