My guess is that it is either a hybrid or could of had a genetic mutation from inbreeding like some deer that are albino or marble collored.
If it is a hybrid mallard-moscovy mix, of all the better looking ducks there are why did it have to choose the ugliest.
It would be kind of neat if ducks like pintails, wood ducks, and other awsome looking species could cross breed to create a duck that would be cool to shoot. Or if ducks and geese could cross breed to make ducks bigger to make for an easier target.-lol