[I got exactly the answer I was hoping for, and shockingly quickly.]
I don't know whom else to turn to:
Why is Scrooge McDuck's butler named Duckworth, even though he is an
It's a good question, and I don't have a conclusive answer. Duckworth was introduced in Duck Tales, I believe - he didn't appear in the Carl Barks comics. In the world of the Uncle Scrooge and Donald Duck adventures, there's an unusual distribution of animals - almost all of the main characters are ducks or other sea birds (geese, the occasional pelican, etc). But the large majority of people are dogs. There's also a minority of pigs. The dogs may be to help distinguish the principal characters from the mass of extras, although some major figures are dogs - the Beagle Boys, Scrooge's one time mentor Theodore Roosevelt, Duckworth, etc. The pigs, when they do show up, tend to be villains, although I don't know that that's universally true. Some of us were trying to work through whether there's some weird ethnic agenda behind this, but if there is, it's certainly not clear - ducks and other birds appear as villains, poor people, the uneducated, etc, as often as not. Ducks, dogs, and pigs never object to treating each other as peers where it would be appropriate.
The Duck adventures are ostensibly set in the same world as Mickey Mouse's adventures. Duckburg is taken to be in the fictional Pacific coast state of Calisota; I believe Mickey Mouse's hometown is, as well. But a Mickey Mouse mystery can involve cows, horses, and pretty much any other sort of animal, whereas the Duck stories stick very closely to the dog/duck/pig mix. When Scrooge and the boys go to Greece to get the Golden Fleece, they encounter harpies where the human half is an anthropomorphic dog. In Duck Tales, there are a couple instances of Scrooge mentioning "human beings" to refer to all of the anthropomorphics, which is kind of weird.
As to Duckworth's name, I guess they were drawing on the same butlering tradition that gave us Wadsworth in the film Clue, and it reminds us to think of how much that duck (Uncle Scrooge) is worth. A quick Wikipedia search reveals that there are some real world people named Duckworth as well. I guess in developing his character, the cognitive dissonance of a dog named Duckworth was overwhelmed by the aptitude of the existing name Duckworth for any butler of Scrooge's.
It's hard to imagine Duckworth as anything but a dog. One thing about the dogs in the duck stories is that I've seen Aboriginal dogs who had dark skin, whereas most of the dogs have caucasian colored skin. Which is weird, since real dogs are covered with hair and aren't the color of white people. With the pigs, it's less of an issue, since pigs basically do have the same skin as white people. I wonder if Duckworth's first name was ever given...