Not sure if this is quite the right thread but I was thinking about Phione earlier and the endless debate over whether it's a mythical Pokemon.
Back when Diamond and Pearl were released, there was a lot of debate over whether Phione, Manaphy's odd little sort-of child, was supposed to be a mythical Pokemon or not. Up until that point the criteria for being a mythical was more or less straightforward (even though the distinction wasn't formally made until later) - there had only been four* mythicals before Gen IV, all of which kept to fairly rigid rules (a BST of 600, not catchable in-game).
The four new mythicals in Gen IV kept more or less to this standard, although Arceus did have base stats exceeding the previous definition. But no-one knew what to make of Phione. Stat-wise it wasn't super, players could hatch an infinite number of them from eggs, and it's literally never mentioned in any main series game. It also, um, doesn't have any real myth or lore associated with it beyond "it's sort of like Manaphy but not really".
But despite this all evidence points to it being mythical. It's never been available normally (except in My Pokemon Ranch in which players can trade for it), is never required to complete the Pokedex, and despite it being no stronger than Glalie, no battle facility allows the player to use it. Official sources were confused, with Pokemon.com once answering "yes" to the question before hastily deleting the answer, and various guidebooks stating alternately that it was and that it wasn't.
However, if it were added to the series today, I don't think anyone would be in any doubt as to what Phione is meant to be. Until Gen VII, the rules for being a legendary Pokemon were mostly quite easily defined. They're strong, not obtainable through ordinary means, almost always unique, and are always single-stage even if capable of changing form (Professor Rowan even states in DPP that "legendary Pokemon don't evolve"). Now the rules have been thoroughly upended; we have legendaries and mythicals which can evolve, are demonstrably not unique, and are far weaker than their legendary status should imply. Phione was simply ahead of its time, and the only reason there was doubt was that it was the first to break the established rules.
I'm personally completely convinced that Phione is a mythical, but does anyone think differently?
*Yes, Ho-oh and Lugia were treated in the same way as mythicals in Gen III, even though they're regular legends, but that was only for those specific games since they were locked behind events and it would have been unfair otherwise (side note: it's kind of funny how Gen III did that given how it would be unthinkable nowadays to not be able to catch old legendaries in new games).