[tveita]

There are real examples of males and females being initially mistaken for different species, as well as for adults and juvenile forms.

e.g. https://en.wikipedia.org/wiki/Cetomimidae

  In early 2009, the Royal Society published an article detailing the discovery "that three families with greatly differing morphologies, Mirapinnidae (tapetails), Megalomycteridae (bignose fishes), and Cetomimidae (whalefishes), are larvae, males, and females, respectively, of a single-family, Cetomimidae."

[thaumasiotes]

In this case the males and females "actually are" different species by an objective definition, though not by every objective definition.