Unusual molting? Hybrids? Obscure species?

I am challenged by many groups of birds. Hawks. Sandpipers, peeps, and assorted small shorebirds. Gulls.

But nonbreeding ducks lead the list.  Seeing these with a flock of Mallard Ducks in Billings, Montana, last week led me to speculate that I was perhaps the discoverer of a whole new species of ducks. Or maybe not. You be the judge. And if you can name any or all of these, please do.

Given the white geese in the above shot, it is possible that there are tame ducks in this waterbody also, but the tame ducks I used to raise were white. Unless they mingled with the Mallards and presented us with these delightful variations.

This last duck does not bear any resemblance to a Mallard, mutated or not, but I still don't know what it is. Quite possibly an ordinary duck wearing its winter colours.