What's in a name — Maned Duck or Maned Goose?
Life is a gradient of relatedness and what taxonomy tries to do is put it in that order.
When the common names were thought up for waterfowl they were either called a duck or a goose according to how we traditionally recognised each — ‘ducks' generally being smaller and shorter-necked than ‘geese', with a broader bill evolved for an omnivorous diet; ‘geese' tend to be more terrestrial, with a stronger bill evolved for grazing. In the case of the Maned Duck/Goose, it seems a consensus couldn’t be reached!
Features we attributed to each group sometimes came about independently through convergent evolution. Thanks to DNA sequencing revealing more about their relatedness, we now know that these two groups are paraphyletic, so the common names can now cause confusion. This can be avoided by just using the taxonomic names instead.