What do you think Nidoking and Nidoqueen are based of?
Their lines seem to be inspired by bits of all sorts of animals - we git some unmistakably mammalian influences, for example from rhinos, rodents/rabbits etc. but also something archaic and reptilian - hey don’t have cheeks for example. I think that they form a unique little clade within pokémon, a distinct body plan (that includes having also similar quadruped pre-evos) that allies them with the rhydon line. Indeed the Nido royalties, Rhydon, and Rhyperior all are known as The Drill Pokémon. Also, Khangaskan shares many features of this group too.
The body plan of a large, eared, bipedal, tailed, spiny, reptiley-mammal has an unmistakably gen 1 feel about it, it is so recognisable, for example it stars as the iconic gym statues, and as one of the major roster sprites in gen1/2. Rhydon like mons everywhere man.
Of course, rhydon was one of the first Pokémon designed, and you can see from this early concept art that the original rhydon was even more Nido like than now.
Ok I don’t know where I’m going with this, but before I had a tendency to shoehorn pokémon into definitive groups based on real life counterparts, however this often runs into difficulties where you have pokémon based on more than one unrelated real life animal, or when a real life counterpart simply doesn’t exist. Plus we have lots of game mechanic bullshit that can be just plain biological sacrilege - eggs, B̑́ͧ͐rͨͩ̇ͣ̏ͧE̷͗E͑͟d͗͆iͤͦͫ͒n͟Ǧ̃̃̄̌, evolutionary stages (though I see pokémon evolution in this sense as a form of facultative metamorphosis) etc.
I think now it’s better to assume that Pokémon is an incredibly different universe, but one that does share some fundamental biological and evolutionary laws and similarities. Thus for example instead of saying that Rattata is a rat definitively, it could be better to say that Rattata is an organism that shares many morphological and ecological characteristics with a rat, and thus, since we have very limited data to go upon (we can’t properly study it’s physiology, behaviour etc., sequence DNA etc., and like,the pokedex is written by a ten year old child lol), we can infer aspects of it’s biology from rats as well as it’s evolutionary relationships to other rodent like pokémon that share a similar suite of physiological and ecological characters, from what we know about such relationships in Rodentia, and from real world character evolution.
So what can we say about the Nidos? They are part of a distinguishable group of pokémon that all possess a distinct set of ecological, behavioural, and morphological characters, which have analogies to a variety of real life mammalian and reptilian real life counterparts, such as rhinos, glires (rodents and lagomorphs (hares, pikas, and rabbits), and mammal like reptiles. The variation within this group of pokemon is less than the differences between members of this group and other pokémon, and this unites this group as a clade. It would be a lot of work to actually work out what pokémon the drill pokémon + khangaskan are most closely related to, i.e. make a phylogenetic tree, but here is just a rough within group tree with a possible scenario of major character evolutionary events (indicated by black rectangles).
(also, the breeding rules between the Nidos, and whether the king and queen are different species or this is just sexual dimorphism is completely clouded by game mechanics of having gendered pokémon before established gender mechanics, so I’m not going to into that - however, I will say for a quick example, meerkats live in a matriarchal society and the alpha female usually has most of the breeding rights - however, because she has to defend the her position, as well as the group itself from danger, she often has very high testosterone levels which reduces her fertility - perhaps nidorina and nidoqueen are extreme examples of this, changing physically to take on a more protective role, letting nidoran sisters/aunts/mothers take on breeding roles)