While I cannot immediately provide a citation, I would guess that in most markets one cannot buy mutton in the U.S. because U.S. culinary tastes do not consider it “good eatin’” and there is simply not a sufficient market to establish a steady supply. (Horse, dog, and a few other critters that are eaten outside the U.S. fall into the same category.)
The USDA does have standards for mutton (with the lamb standards) (.pdf) and not, that I can find, for horses, indicating that some mutton must be sold in the U.S., but it would probably be a specialty market in regions where traditional local custom or ethnic tradition supports enough sales to make it worthwhile.