"Why do mammals all have different names for their meat? Why do we eat beef, venison, and pork instead of cow, deer, and pig?"
After the Norman conquest of England, the whole aristocracy in England were French-speaking Normans, and English had become the language of farmers and peasants. So the words for the animals in the field all stayed English ("sheep"). But when the animals showed up on the table of the nobleman who could afford to eat meat, they became French: ("mutton"/"mouton").
Pretty cool linguistic question for a kid.