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.

Cheers, Sören

Later addition: "Billy the Bastard" would indeed have been an appropriate and probably undisputed name for the young William, I learned.