Okay, so what counts as "Mythical" now?
I put forward that what is and isn't Mythical changes depending on the era.
Generation I: +Mew
Generation II: +Celebi
Generation III: +Lugia/Ho-Oh/Jirachi/Deoxys
Diamond/Pearl/Platinum: +Phione/Manaphy/Darkrai/Shaymin/Arceus
HeartGold/SoulSilver: -Lugia/Ho-Oh
Generation V: +Victini/Keldeo/Meloetta/Genesect
X/Y: +Diancie/Hoopa/Volcanion
Omega Ruby/Alpha Sapphire: -Deoxys
Sun/Moon: +Magearna/Marshadow
Ho-Oh and Lugia didn't start as Mythical, but counted as Mythical from Generation III to Platinum because they were only obtainable through events/distributions/side games and weren't required to complete the National Pokédex.
Deoxys was Mythical from its introduction until Omega Ruby/Alpha Sapphire where it became possible to catch one through normal gameplay. It was also not counted with all the other older Mythical Pokémon during the 20th anniversary distributions.
Phione is Mythical because it's not required to complete the National Pokédex. Although it wasn't part one of the distributed Mythical Pokémon during the 20th anniversary, it was most likely due to it being available through Manaphy. So while it's Mythical (and Mythical =/= Legendary), it's not of the same rarity as other Mythical Pokémon.
