Not saying I agree, but I found this piece here. It also should be kept in mind that the arrogance is just a joke:

Alright, look: I know you've all comfortably thought of these two Pokémon as "ducks" since the dawn of the 21st century, but you're just going to have to wake up and accept the fact that having "duck" in their names doesn't mean a damn thing. During the video game's earliest prototype phases, "flying" type was known as "bird" type, and assigned to every bird-based Pokémon whether or not they could actually fly (see: Doduo and Dodrio). Psyduck and Golduck, however, did not receive the "bird" or "flying" type because they are not birds. With four webbed limbs, a full covering of fur, mammalian tails and a habit of swimming beneath the water, these guys always have been and always will be duck-billed platypses and there's not a damn thing you can do about it. You might try to say that they're just stylized, duck-like monsters, but that would still make them stylized, duck-like monsters identical to platypuses. You must see the futility in arguing with me.

Platypuses are egg-laying mammals or "monotremes," an ancient group survived only by these animals and the echidnas or "spiny anteaters." Their duck-like bills are sensitive to the electrical fields of their prey, and males are armed with a highly venomous spur on each hind foot.

Yes, I'm also aware that Golduck is intentionally similar to the (typically more reptilian) Kappa of Japanese folklore, but this is an article about the natural world, not the supernatural, and Golduck clearly has a primary animal basis.