Golduck evolves from Psyduck starting from level 33, then progresses to DaffyDuck at level 60. For some reason its name is Golduck when its unevolved form is golden but it is not. Presumably this is the same reason that Psyduck is called Psyduck when Golduck is the one with Jemm's telepathic forehead-bead.
It can be safely assumed that the naming conventions here were due to the same sort of inter-departmental pooch-screwing which resulted in Ducklett evolving into some swan crap no-one cares about instead of shuffling in at the beginning of this evolutionary chain.
Much has been said about the possibility of Golduck being based on the Kappa from Japanese folklore, which is just plain stupid, because does that look like a duck to you?