Why does canis have both masculine and feminine forms?

Question

Most nouns in Latin (and e.g. Spanish) have only one gender. Some other have two (epicene nouns). canis is one example (Separate Q: are there more examples?)

I wonder why is that the case for canis. My naive guess is that it is because of their proximity with humans. That is at least what I get from Spanish. In effect, in Spanish dog also has both cases (perro y perra). Same with cats (gato y gata), whereas in Latin cat is only feminine. Interestingly, most of other animals in Spanish only have one gender, suggesting to me the reason for this is precisely because of their proximity with humans (related question here).

However, in Spanish the words are different, whereas in Latin the word is the same. So maybe the reason is different.

Answer 1

Most Latin animal names have both a male and a female form, to express the animal's gender; the masculine is used when the gender is unknown.

If the masculine form is second-declension (e.g. "ursus"), then the feminine form will be first declension ("ursa"), since second and first declension nouns are almost always masculine and feminine, respectively.

However, third declension nouns (e.g. "canis") can already be either masculine or feminine. So there is no need to construct a separate feminine form. That said, the word still has either a masculine or a feminine gender, despite the two forms being identical, as shown when an adjective agrees with it (e.g. "canis laetus" versus "canis laeta").

The same happens with adjectives. First/second declension adjectives will have separate masculine and feminine forms (e.g. "laetus," "laeta"), whereas for third declension adjectives the masculine and feminine form is the same (e.g. "fortis").