[thaumasiotes]

> The only way to breed a Tauros is via ditto, same as with any other male-only species.

You're half right. I was wrong about Tauros and Miltank.

But Volbeat and Nidorino are male-only species that don't need to be bred via Ditto. You can breed Volbeat with Ditto and get an Illumise. You can breed anything with Illumise and get a Volbeat. You can breed anything with Nidoran-female and get a Nidoran-male. You can breed a Nidorino with Ditto and get a Nidoran-female.

Outside-of-universe, this is obviously because the technology of representing Pokemon within the game has changed over time. Later male-female pair species are given the same name and the same Pokedex number. But within-universe, that explanation isn't available.

[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.