Why does canis have both masculine and feminine forms?

1. What declension is canis canis?
2. What is masculine and feminine in Latin?
3. Is animal a common gender?

What declension is canis canis?

Third-declension noun.

What is masculine and feminine in Latin?

There are three genders in Latin: masculine (masculinum), feminine (femininum) and neuter (neutrum). The gender of the nouns is natural, i.e. in accordance with their sex (especially if they are living creatures), or grammatical, i.e. in accordance with their terminations.

Is animal a common gender?

If it's a domestic animal, then it's common gender, otherwise it's grammatical.