People wonder why Psyduck isn’t a Psychic-type, and I do too, but why isn’t it a Flying-type? It’s a bird, and all other generation I birds are part Flying-type (including Farfetch’d, who is also a duck, and Doduo/Dodrio, who are also flightless).
I think this may in fact be the best argument for thinking that Psyduck is not a duck at all, but a platypus, which I know people have suggested. There’s an important caveat here that even at the best of times Game Freak doesn’t quite seem to know what the Flying type actually is, and frankly neither do I, but other than the beak there’s not a lot which is especially ducklike about Psyduck. He has arms and webbed hands rather than wings (likewise Golduck), learns no Flying-type or bird-themed attacks aside from Aerial Ace (which… look, just f&$% Aerial Ace), and his body doesn’t really look feathery the way most bird Pokémon do.