Allen and Greenough's New Latin Grammar, section 34

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34.

Many nouns may be either masculine or feminine, according to the sex of the object. These are said to be of Common Gender: as, exsul, exile; bós, ox or cow; parens, parent.

NOTE: Several names of animals have a grammatical gender, independent of sex. These are called epicene. Thus lepus, hare, is always masculine, and vulpés, fox, is always feminine.