Muscovies are not a breed but rather a species.
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Scientists have produced fertile hybrids, invariably males, in experiments (according to published reports, personal conversations and unpublished notes of scientists) with related species including Spur Winged Goose, Maned Goose, White Winged Duck, Brazilian Duck, Steamer Duck, and Mandarin Duck. I don't know of any recent attempts save the Australian Pygmy Goose hybrid that resulted in fertile offspring and that was via artificial incubation. Some small percent of male offspring evidently proved at least partially fertile. I remember also reading about an experiment in Japan breeding Mandarin ducks with other ducks to determine relatedness. Hybrids with the closely related Wood Duck were only generated a very few times and those hybrids never attained male plumage and may have been sterile I don't recall- but strangely hybrids, again via artificial insemination with the muscovy and I think a shelduck of some species produced a small percentage of fertile male progeny- I remember being surprised at the level of fertility of eggs of the Mandarin X Muscovy cross but don't recall how many actually hatched or what percentage of the males were fertile- I've never heard of a second generation hybrid other than with the Comb Duck and White winged duck- but don't know that this has ever been an objective of the scientists. Other hybrids with these related waterfowl species have occurred naturally in captive situations and the hybrids never bred but that doesn't mean they were necessarily sterile.
Anyway- the muscovy cross if one were ever produced- that was a "bantam" , for example White Mandarin X Muscovy would theoretically be bred back to muscovy and selectively bred for generations. By the time fertile females were being produced umpteenth generations later I think they'd be considered muscovies I would imagine. They may well be of hybrid origin themselves. Authors have suggested the Comb Duck may be a second paternal ancestor of the first domestic muscovy.
I know certain domestic dabbling ducks have wild dabbling duck species other than mallards in their genetic makeup and I think we would all just assume they were descended of the mallard and as such domestic mallards. All those dabbling ducks are fully fertile when crossed so that's problematic to begin with. I read somewhere in Japanese literature that molecular data revealed that the East Indies duck is the product of Philippines Duck X Mallard hybrids. Similarly Indian Runner Ducks and some other old Asiatic breeds have other species of dabbling ducks involved in their genetic makeup- these ducks all resembling female mallards with no difference in plumage between the genders. They've all been selectively bred for so many hundreds of years it's difficult to imagine them even having wild ancestors and their genetics are probably in the bloodlines of more modern breeds like the Cayuga and Khaki Campbell respectively.