And heck, even then there's exceptions, like Galarian Yamask (it's unclear whether its a Yamask that got cursed by a runestone or it's a runestone that attracted a spirit and just so happened to form into a Yamask).
See, to me it doesn’t seem unclear at all — Sword’s Pokédex entry for Galarian Yamask states that a cursed clay slab took possession of a Yamask. Shield’s entry is a little more vague, saying that the slab was drawn to a “vengeful spirit,” which I suppose could be any old unattached spirit, but given the context of the Sword entry, as well as the larger context of how regional forms have been defined, I think it is reasonable to take that entry as simply referring to an existing Yamask.
So when I see something like Poltchageist, the distinction between phenomena is reasonably apparent: a regional variant is an existing species that adapted to a new environment (or was changed by something unique to that environment), while a convergent species is a creature that coincidentally took on a form similar to that of another unrelated creature.
Voltorb and Hisuian Voltorb can no way be related.
Why not? Voltorb’s own origins have always been kept nebulous; I think there’s ample room for a connection with Hisuian Voltorb to be made.
> “It was discovered when Poké Balls were introduced. It is said that there is some connection.”
> “During the study of this Pokémon, it was discovered that its components are not found in nature.”
> “Voltorb was first sighted at a company that manufactures Poké Balls. The link between that sighting and the fact that this Pokémon looks very similar to a Poké Ball remains a mystery.”
> “Voltorb is extremely sensitive - it explodes at the slightest of shocks. It is rumored that it was first created when a Poké Ball was exposed to a powerful pulse of energy.”
With a little creativity, there’s a lot of room for association here. Maybe a Hisuian Voltorb found its way into one of those Poké Ball companies, wound up inside a Poké Ball, and then that Poké Ball was “exposed to a powerful pulse of energy,” causing the Hisuian Voltorb to mutate. Or maybe it was just some of that Hisuian Voltorb’s seeds that got inside some of the Poké Ball supply and were exposed to the energy. Or maybe that rumor is just a rumor, and Hisuian Voltorb mysteriously transformed itself to better match the new kinds of Poké Balls. Maybe Voltorb is some kind of Absorbing Man-type that can take on the properties of whatever it comes into contact with. I mean, we’re in the series with a sludge creature formed by x-rays from the moon!
what I'm saying is that it doesn't even meet the qualifications to be a regional varient
In the sense that there’s only one extant specimen, sure. But if you were to bring
more Ursaluna into the Timeless Woods, what would happen? The precise factors needed to cause regional variants to develop are already about as vague as what happened to that one Ursaluna, so the distinction is murky at best.
In fact, the phenomenon behind Bloodmoon Ursaluna seems only a short distance away from the phenomenon that produced Hisuian Decidueye, Typhlosion, and Samurott — all of whom were the result of singular specimens being brought to the Hisui region. Presumably, they were not genetically any different from other Rowlet, Cyndaquil, or Oshawott, and yet, simply by evolving within Hisui, they matured into entirely new forms adapted to the environment.
I’m not trying to push for a formal reclassification or anything, of course. My point is mainly just that, in the same way it was proposed by
Pikachu315111 that Poltchageist and Sinistcha may have been designed as convergent species in order to circumvent the terminology of a “Kitakamian Form,” Bloodmoon Ursaluna may have been designed under a similar logic — something that is thematically not that different from a regional form, but is just different
enough to where the established regional form terminology can be eschewed.