Epicene

Q. How will you diſtinguiſh the Maſculine hic from the Epicene hic, and the Feminine hæc from the Epicene hæc? / A. That word that hath hic before it, and is onely male, is the Maſculine gender: but if it be both male and female, then it is the Epicene Gender: and ſo hæc before a female, is feminine, but hæc before a word that contains under it both ſexes, is Epicene.

Source: wiktionary

Epicene nouns are equally misunderstood: they are of one gender only. These, like the common, represent under one word each member of a pair of animals—the male and the female: thus passer—a sparrow—denotes the cock sparrow, as well as the hen: but in the use of these words there is no variation of the gender: they are invariably used in one gender only: thus passer is of the masculine gender: and though used for the purpose of representing the hen-sparrow; still every adjective or participle connected with it must be used in the masculine gender: [...] In short, epicene nouns differ from the common in this only; that they do not vary their genders in accordance with nature: they invariably keep to one gender.

Source: wiktionary

In many names of animals, the same word with the same gender is used for both sexes: ἡ ἁλὠπηξ the fox, male or female. These are said to be epicoene.

Source: wiktionary

Ovis, therefore, is epicene, and, moreover, a true epicene, like volpes, aquila, merula, avis, panthera, corvus, and others. It is epicene, because it has just been proved to be the generic term for sheep without thought for sex, to have only one grammatical gender, feminine, and yet, as a true epicene, to be carried to its logical development, so that on a few occasions, such as we have encountered in Ovid, and in [Marcus Terentius] Varro, it is employed strictly of the male.

Source: wiktionary