Well, the Pekin breed gets that name because that particular breed of domesticated Mallard is the raw material for the famous Peking Duck. Muscovy is, I'm pretty sure, a different duck species, whereas Long Island is another domesticated Mallard breed.
I suspect that most Chinese markets offer one of the domesticated Mallard breeds. But I can't be sure. I usually consider myself fortunate if I can identify the animal of origin and piece of anatomy of meat in an oriental market.