The word mammal typically refers to vertebrates of the class Mammalia.
There are no categorical antonyms for this word.
However, one could loosely refer to animals of any unrelated class as antonyms, e.g., birds (Aves), reptiles (Reptilia), or amphibians (Amphibia).
Opposite of plural for a human being
“He gobbled that steak up like a wild animal.”
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