lamb (which comes from l'angneau),
the choice of the word lamb curiously breaks that schema (which stays valid nearly everywhere else). It is believed to be a much older word from germanic/protogermanic roots, qf the German "Lamm", Dutch & Scandinavian languages "lam", Nedersaksisch "Laom" etc.
Just the other way round mutton is from French mouton, so you have been unhappily mixing the pair the wrong way up.
Later addition: "Billy the Bastard" would indeed have been an appropriate and probably undisputed name for the young William, I learned.