There are several concept of animal or plant in real world that has never been put into pokemon, namely what i know is:
1) Hermaphrodite Pokemon ( Some say ditto is the closest,but still not sure)
2) Endangered Pokemon, im not talking about legendary pokemon, what i mean by endangered here is that, these pokemon is REALLY hard to found in the wild, and its population is the lowest.Because of its low population, you can only get it via ranger or Pokemon Association who protect these endangered species. You might think like Iriomote mountain cat, which only found in the Iriomote island, but i heard that its already been put through, is it true?
3) Bacteria or Small insect like mosquito, well i always want to look a joke that Ash and his friends got sleep problem, because of many mosquito, i wish they put those too, maybe swarm of mosquito
That's what i think that never been been put in pokemon, any new concept to these? Please say so..