How tf is phione a legendary but not Volcarona, Rotom or Spiritomb????
Well, there is no real definition of what a legendary Pokémon is, other than “the ones we say are legendary Pokémon.” It seems to me like the distinction has two parts. There’s a real-world reason, related to how you, the player, go about obtaining the Pokémon practically, and there’s an in-universe reason, roughly related to how well-known the Pokémon is.
To us, Phione is a legendary – or, more specifically, “mythical” – Pokémon because it’s impossible to get it just by playing the core games of generation IV on your own (you need to get Manaphy from an external source, namely a copy of Pokémon Ranger, or in later generations from any of several event distributions). Other legendary – but not mythical – Pokémon are available entirely in-game, but usually only one of each per save file. Even single-encounter Pokémon like Volcarona, Rotom or Spiritomb can later be bred to get more than one.
To people in the Pokémon world, Phione is a legendary Pokémon because of how little they know about it – which I think is an interesting distinction, and one that feels a lot weirder than it once did because of how Pokémon’s successive titles all relate to each other. Back in Red and Blue, there were four “legendary Pokémon”: Articuno, Zapdos and Moltres, the “winged mirages” whose existence was kind of known but hadn’t been scientifically confirmed, and Mewtwo, a one-of-a-kind genetic experiment who was created recently in a project that was not public knowledge. And then there was Mew, who was a very special case, because Mew wasn’t even supposed to be in the game at all. But we know a lot more about all of those Pokémon now – and not just us, characters all over the Pokémon world. They’ve now been able to study Articuno, Zapdos, Moltres and to a lesser extent even Mewtwo for a very long time. Because the core Pokémon games are very averse to changing things that were established by previous titles, those Pokémon are all still “legendary,” but… well, why would they be? They’re still very powerful, certainly, but I’m not sure that’s all that being a “legendary Pokémon” was ever supposed to mean. I think it would make a lot of sense if more information about those Pokémon had gradually come out of the domain of myth and folklore and into the domain of science over the years – which you could even represent mechanically, if you wanted to. For instance, maybe we understand how the legendary birds’ mating habits work now, and we can breed them when that wasn’t possible before.
Which leaves us with Phione. Obviously Volcarona, Rotom and Spiritomb are all the subject of a lot of mythology, folklore and/or rumour, just like Manaphy and Phione. There’s just some invisible line somewhere where there’s just a little bit more information about Volcarona, Rotom and Spiritomb that we know for certain, just enough to bring them out of the “legendary” category and into the merely “very mysterious” category. Thinking again about the fact that Manaphy doesn’t naturally exist in Sinnoh, we could suggest that maybe these distinctions even vary by region; Manaphy and Phione might not be truly “legendary” to Fiore, but Sinnoh’s scientists have never seen one, so they’re much less confident in their information about them. Which is fine as it goes, in Manaphy and Phione’s original context of generation IV – but intuitively, surely we know a lot more about them by now? Surely Phione can’t still be more “mythical” now than Volcarona was in generation V. And so the disconnect gets wider and wider over time…