Why do nidoran evolve differently based on gender, but other pokemon which experience relatively extreme gender dimorphism (such as frillish) become the same pokemon no matter what their gender? What sets nidoran apart?

Strangely, the nidoran lines are the only pokémon lines whose male and female counterparts … are not the same species. That is, although the two get along well enough, they’re about as similar to one another as plusle are to minun. The only reason they’re both called nidoran is because they look like one another, often form mated pairs (even if the advanced stages can’t breed), and live in the same burrows, so scientists mistook them for the same species with a divergent evolution tree (much like the burmy line) until later studies revealed that they’re only related, not the same.

So that should answer the question about their evolution lines: they are, quite simply, actually two separate lines from their base onwards.

Hey there Bill, I’ve had both a Nidoran Male and Female since I was little. I grew up with them by my side and they grew fond of each other. Now they’ve evolved into Nidoking,and Nidoqueen and I was hoping to breed them to get a new generation of Nidorans. I’ve hit a snag though, it seems from whatever reason, they can’t breed. I went to a few other experts and they told me that Nidoqueens can’t breed with Nidokings, and i was wondering if you had any idea why?

By and large, nidoqueen and nidorina are incapable of breeding because for whatever reason, evolution sterilizes female nidoran. Male nidoran and their evolutions, meanwhile, retain the ability to breed throughout their lives, and thus, male nidoran often mate with either female nidoran or ditto to perpetuate their species; they don’t actually mate with a female nidoran’s advanced forms.

As for why evolution sterilizes female nidoran, that’s still a subject of debate in the scientific community, but to be fair, considering the fact that male nidoran grow longer, sharper, and more numerous barbs upon each subsequent evolution (including and especially exactly where you would think), one can hardly blame nature for wanting to offer female nidoran a bit of mercy, if you get my meaning.

Any notable encounters with the Nidoran line, Bill?

Ah, the Nidoran line. This may be a bit complicated because the male and female lines are so vastly different from one another that they’re officially considered to be separate species, so I’m afraid I’ll have to cover them on two separate days. Below is the first, and tomorrow will be the second.

Nidoran (female)
The Poison Pin Pokémon
Type: Poison
Official Registration #: 29
Entry: A small, rabbit-like pokémon. An interesting fact about nidoran is that the first pokémon ever to be recorded using the move Poison Pin were members of this and its brother species, hence why they’re both described as “poison pin” pokémon in the pokédex. Of course, the story of how Poison Pin was discovered is equally interesting. Evidently, male nidoran are very protective of female nidoran, and female nidoran have a tendency to fire Poison Pin in tandem with their mates when alarmed.

Nidorina
The Poison Pin Pokémon
Type: Poison
Official Registration #: 30
Entry: The evolved form of female nidoran, by battle experience. Upon evolution, nidorina and their brother evolution lose their ability to breed. However, make no mistake: nidorina still possess mothering instincts—such as, specifically, the tendency to unsheathe their barbs and emit ultrasonic cries when anything threatens their burrow and young. As a note, what the pokédex says about nidorina’s docile temperament is true except in the above mentioned case, but the trouble with pokédex entries is that they never accurately describe how frequently exceptions to the rule occur. For example, in this case, yes, nidorina are docile, but it is highly useful to know that all routes where nidorina thrive are literally covered in nidorina burrows. Thus, when traversing routes through nidorina territory, bring antidotes. A lot of antidotes.

Nidoqueen
The Drill Pokémon
Type: Poison/Ground
Official Registration #: 31
Entry: The evolved form of nidorina, by exposure to moon stone. Nidoqueen are large, bulky pokémon covered in rock-hard plates. Both nidoqueen’s size and skin are highly useful in defending its young from threats, which it does with pride and enthusiasm. On the positive side, moon stone is very rarely found in nidorina habitats, making nidoqueen a very rare evolution to find naturally. However, on the negative side, nidorina territories really only need one nidoqueen to defend them, and that nidoqueen very rarely travels far from human roads. Luckily, hospitals are likewise very rarely far from nidoqueen territories.