> 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.