Some Latin nouns are common gender: their grammatical gender varies depending whether they refer to a male or a female (human or other animal). This is mentioned in many Latin grammars (including Allen and Greenough), and examples are often listed. However, I have never seen any proof in grammars that any of the listed words really behave this way. I don't really doubt that common gender nouns exist; I just found myself unable to very convincingly argue that they are a real thing in classical Latin.
To remedy this, I would like a couple of example words (two is enough), each word with two example passages from classical literature: one where it is feminine and another one where it is masculine. It would be great if the examples would be relatively simple and, most importantly, gender-wise unambiguous. Which example words and which passages would you suggest to demonstrate the existence of common gender nouns?
Notice that the question is not about all nouns with varying genders, but those nouns whose grammatical gender depends on the biological gender of the referent. This rules out words like dies and locus (with split plural loci/loca).
This question was inspired by an earlier question about feminine homo.