While Miltank could be only female, I’d make it so Tauros could be male or female in the wild.
Miltank would be a potential phenotype bred into the population by humans. Like aurochs the orginal wild tauros would haveextinct, and were replaced by feral tauros with the potential to give birth to miltank in the right environmental conditions.
This would also work with how miltank looks like a dog in a cow suit; a sort of joke on how domesticated mammals develop similar traits and the “all animals are dogs” joke.
To get a female tauros from breeding you’d need an incense though; miltank appear in captivity when breeding for females otherwise.