Today I Learned: Arceus is not considered a Legendary Pokémon, but is instead categorized as a Mythical one. These labels are mutually exclusive, and which way Pokémon fall is quite unintuitive. Mew is Mythical, but Mewtwo is Legendary. Alien Pokémon Deoxys is Mythical, but alien Pokémon Eternatus is Legendary. Cresselia is Legendary while Darkrai is Mythical, despite the two having so much in common they are referred to as the Lunar duo. The lake guardians (Uxie, Mesprit and Azelf) are Legendary, but the sea guardians (Manaphy and Phione) and the forest guardian Celebi are Mythical. The trio of Dialga, Palkia and Giratina are referred to as Pokémon of myth, but are all Legendary Pokémon.
I think my confusion may stem from the recent distinction; my obsession with Pokémon lasted until the release of Generation V, which coincides with the formalization of the Legendary/Mythical split, prior to which only the Legendary label existed (and so Arceus, Darkrai etc. would have indeed been Legendary).
Most of this information comes from the Bulbapedia article on this and it has lots of cool trivia, including some inconsistencies in the use of the labels over the years. There are very many such cases but some of the more interesting ones are:
- Arcanine is sometimes referred to as Legendary (and even has a listed category of "Legendary Pokémon") despite not even being a pseudo-legendary one
- Unown has been referred to as Legendary in official media.
- Aforementioned sea guardian Phione's Mythical status is often in contention.
- Delibird and Rotom have both been erroneously published as Legendary (there were many mistakes in the book though)
Throughout all this, I have not seen an explanation of what makes a Pokémon either Mythical or Legendary. I thought there was a heuristic of the "main" of core games always being Legendary (with the exception of the Generation I games), but with the Let's Go! games and Legends: Arceus being considered core games, this is no longer valid. In researching this post though, I came across the concept of game mascots, for which this works better, as the Legends games do not have box art mascots, and so the only exceptions are obviously neither Mythical nor Legendary (evolved starters and partner Pokémon). Of course, none of this matters very much, and I doubt even the Pokémon community really cares (the Bulbapedia pages on these subjects are great, but I did notice some grammatical errors and inconsistencies with other pages), so it's unlikely there will ever be consistency for this.
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