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?
This question was inspired by an earlier question about feminine homo.