>>29928814"Mythical" according to the trends currently being followed simply refers to an Event-only Pokemon, meaning you can't catch it by any normal means until a special code or some other distribution is held.
Deoxys used to be Mythical but now you can get it in-game without any special events, so now it's simply Legendary.
In-game, Mythical means almost nobody in recorded has ever confirmed whether the Pokemon is real or not (until you, the player, happens to obtain it, that is).
Currently known Mythical Pokemon are: Mew, Celebi, Jirachi, Manaphy, Shaymin, Darkrai, Arceus, Victini, Keldeo, Meloetta, Genesect, Diancie, Hoopa, Volcanion, Magearna, and Marshadow.
