Nidoran were the first Pokémon to feature gender, and even gender differences; however, their differences are much greater than those of most later Pokémon.
Despite the addition of genders to most Pokémon starting in Generation II, the two variations of Nidoran remained separate species, rather than a singular species that splits down different evolution paths based on gender.
An Egg laid by a Nidoran♀ does, however, have a chance of hatching into a Nidoran♂, as well as an Egg laid by a Nidoran♂ with a Ditto has a chance of hatching into a Nidoran♀.
Nidoran♀ is the only Pokémon that loses the ability to breed upon evolution.
Nidoran♀ and its evolution share their category with their male counterparts Nidoran♂ and Nidorino. They are all known as the Poison Pin Pokémon.
Due to the fact that its gender, ♀, is included in its name, checking Nidoran♀'s summary from Generation II onward will display as if it were genderless if it does not have a nickname. If its nickname is anything other than "Nidoran♀", its gender will appear as normal.
