The original meaning of mutton is an adult sheep. In English I don't believe there is a proper name for goat meat other than "goat meat". In some areas of the Anglosphere mutton is used to refer to goat meat.
Some local curry restaurants in the Bay Area use goat for recipes that call for mutton, and sell it as mutton curry – but I assume that's because mutton is not an option...
If you visit areas with less domesticated animals that forage semi wild with shepards that check in now and again you might struggle to tell the difference between a sheep and a goat - the varieties of each are broader than many people realise and there is a considerable degree of visual overlap in the full spectrum of Sheep Vs. Goat.
Sheep and goats are so skeletally similar that archaeologists can't easily tell the difference. I just googled and there are signs like the third cusp of the third molar etc but I remember an archaeologist telling me that they just refer to them as sheepgoats, as though there was some mystical single type in antiquity.
Great, you get goat meat when you order mutton in the Bay Area? Here where I live, people sell mutton (sheep/lamb) even when asked for goat meat. I don't know why but people here always prefer goat meat to mutton.
