You would also meet it in Renaissance and some Baroque texts, by then spelled goupil and concurrently with renard. Today you meet the word in translations of the Reinardus stories. He's still called Renard le goupil (the medieval spelling was renart li gupil, I think).

For one reason or another, it's one of the archaic words a fair percentage of people know the meaning of (though I wouldn't say Renard is still a popular character, not for many generations on this side of the ocean anyway). It's a common patronym, it's often used instead of renard in business names etc.

I'm not sure why exactly. But if you ask people for medieval words they know, there's a fair chance it's goupil that will come up.

You still hear it (Acadiens are still fond of the first name/à/father's or mother's name formulation; in Québec the last generation that used à for de is all but gone). It still exists in French regional patois. But it's not standard French/proper grammar. It's still exists in the expression "Fils à papa". .

IRRC, those were mostly from the south and were somewhat less intuitive for me (it also probably didn't help my motivation that I had read those texts before in translation). I used the dictionary a lot more to get by. But you've got a much larger knowledge of the other languages from that family than me.