The new Dreepy evolutionary line, to me, brings up some pretty interesting questions about the Pokemon world’s evolutionary biology. They’re apparently aquatic Pokemon that lived in the ancient past that became spirits once they became extinct, and now fittingly have the “ghost/dragon” typing in the modern day. What’s interesting about this is you must wonder, why isn’t this more common, that being why isn’t there a ghost type variant of every existing Pokemon, since all Pokemon are living creatures that shed their mortal coil eventually all the same. This seems to imply that becoming a distinct ghost-type pokemon is something only some are capable of, while the rest just become normal wraiths like Pikachu did in the Pokemon Tower episode and presumably eventually pass on. Might there be some “metaphysical” (or possibly just physical, since this is just how things work in this world) laws that determine how adaptive a Pokemon’s spirit is? And judging what we know of Pokemon that are suspected to have once been the departed spirits of humans (such as Yamask and Gengar) and how different they look compared to humans, how many ghost type Pokemon might be the result of the ghost of a known or unknown species of Pokemon? Hell, if we could somehow find the spiritual version of “genes” would it be possible to trace common ancestry with their mortal relatives, and add ghostly branches to the Pokemon tree of life? The implications of this are both overwhelming and exciting.
Yeah, it’s an interesting problem. I really love the idea of a Pokémon that’s not a fossilised prehistoric creature, like so many we’ve seen before, and instead the ghost of an extinct creature, but it does raise that question – why this Pokémon? Why is Dreepy unique (well, not quite unique; Galarian Corsola seems similar, but they’re obviously unusual)? I would not actually default to thinking that Dreepy became lingering spirits because of something inherent to them, though. In folklore, people become ghosts because of something about the way they died – maybe they have “unfinished business,” or weren’t given the burial rites their culture requires, or were killed by a particular monster, or just died in a particularly unpleasant way that somehow damaged their soul and prevented them from moving on to wherever spirits are supposed to go. In Pokémon, we often aren’t explicitly told where Ghost-types come from, but when we are, my impression is that it’s more often a magical or spiritual cause than a biological or scientific one (of course, then the follow-up question is whether we’re supposed to believe what we’re told about Phantump, Sandygast, etc. or just see it as a mystery yet to be solved). Given what little we know, my first guess would be that Dreepy exist in their current ghostly form due to something about the nature of the event (or competing species, or predator) that drove them extinct. Maybe their species was wiped out by something unusually sudden or traumatic, or maybe there was some Ghost-, Dark- or Psychic-type predator (now extinct itself as well) that could manipulate and damage souls, or maybe – just maybe – they were the victims of some kind of spiritual calamity, like an eruption of the spirit world into the “real” world. That’s the sort of place my mind goes when you raise the question, at any rate. I think in the absence of anything more explicit from an official source, the “correct” answer is probably whatever you feel is the more potentially interesting.