I am using a nidoking and a ditto in the day care and twice I got a Nidoran (girl). I am playing on Pokémon Black 2.
Sine the two Nidoran's Gender determine what they look like, well....it may end up being the completely different Pokemon. That won't always be a Boy Nidoran if you use Nidoking. There's always the chance it'll be a girl.