So, Pokemon breeding mechanics are almost certainly non-canon. But surprisingly, egg groups aren't the issue. Egg groups are reminiscent of sexes. That may seem like an issue, given that there are only two sexes (there is a difference between sex and gender), with some mutations being the rare exceptions, however that is only the case with animals. Fungi are wild and have a bunch of sexes, some having more than 15,000. How this works is very complicated, but essentially, all sexes can breed with any sex but their own. That being said, fungi are very simple organisms, and you all know, Pokemon are complex. Not only that, but Pokemon probably evolved very differently to creatures in the real world, and the Pokemon world probably doesn't run by the same rules as the real world. So any number of small changes could make egg groups scientifically possible. A problem arises however, when you take into account the fact that different species of Pokemon can breed with each other. This is, in fact, something that happens in real life. The difference being that in the real world, if a female polar bear and a male grizzly bear mate, their offspring will be a grolar bear, while in the Pokemon world, if a female Beartic and a male Ursaluna mate, their offspring will be a Cubchoo. Even chalking this up as a difference in the laws of the universe doesn't work, as we start to wonder how Sawk reproduce, as outside of the Friend Safari, they are never found in the same place as Ditto, and since all Sawks are male, the only way for a new Sawk to be created is for it to breed with Ditto. This begs the question, how do Pokemon reproduce in the first place? Well, in the wild, animals usually only reproduce with their own species, even if they are able to reproduce with another. Grolar bears are very uncommon, and are only occurring because of global warming. Granted, polar bears and grizzly bears don't come across each other that often in the first place, but let's look at another hybrid: the wholphin. A wolphin is the hybrid of a female common bottlenose dolphin and a male false killer whale. These two species are found in the same locations, and as such, have been documented in the wild, but it is extremely rare, despite the fact that dolphins are… Well, let's just say we have a lot in common with dolphins. My point is, that while different species do reproduce in the wild, they usually prefer to reproduce with their own species. This logic would likely carry over to the world of Pokemon, as they would naturally evolve to look favorable for their own species. That being said, for Pokemon like Sawk, they might have no choice but to mate with different species. However there is another option. Asexual reproduction, or reproduction with only one parent. This essentially creates a clone of the parent. While uncommon, some female animals can reproduce asexually if they fail to find a partner, and some whiptail lizards only reproduce this way. This may be an option for Pokemon, even male ones, as again, the Pokemon universe works by different rules. In any case, this was important to bring up in order to answer the question of whether or not female Nidoran and male Nidoran are the same species. The answer: I think so. Sexual dimorphism is the difference between sexes of the same species, and occurs in both the Pokemon world and the real world. Some might say that they can't be, because they are listed as different species in the pokedex, however Pokemon in the same evolutionary line are considered different species even though that makes no sense. By our definition of species, Pikachu and Raichu are the same species, and the pokedex is either wrong, or has a different definition for species. Therefore, it is not a stretch to say that both Nidorans are the same species.