You can blame Bill The Bastard (sorry, I meant William the Conqueror) for that. He brought a whole bunch of French words into English. The aristocracy spoke French, the normal commoners spoke Anglo-Saxon. That's why you get two names for most types of meat; mutton and lamb (which comes from l'angneau), ox and beef (from bœuf), etc.
Look closely and you'll find that English (even the version with Merriam and Webster's messed up spelling (color vs colour)) is an amalgamation of Latin, Greek, Arabic, French, German, Anglo, Saxon, Swedish, Norwegian and Chinese plus a whole bunch of other words borrowed from
around the old British Empire.
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.