I heard a theory once that Nidoking/Nidoqueen are based mostly on Rhinos.
Thing is, Rhinos are notoriously difficult to breed in captivity. In the wild they do just fine, but in captivity they're ridiculously stubborn, to the point that even artificial insemination doesn't work, specifically because the female rhinos organs require specific horomones to be able to breed, and they generally just don't come out while in captivity.
Meanwhile, the male Rhinos are fine for the most part.
So, I actually see this as a clever nod to their inspiration: It isn't that Nidoqueen can't breed, it's just that it refuses to do so in captivity, even though it can do so fine in the wild, just like real Rhinos.
