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the officiak terms suck ass because gamefreak and TPC insist on expressing everything in diagetic language to maintain the illusion of a cohesive world. this is fine for official releases, but it's not practical for fans who are aware of the game's inner workings and want to discuss them. fan wikis (like, say, bulbapedia) should stick with the terms that have been used for over a decade now, regardless of if they're "official" or not.
they're event legendaries because they're legendaries only available through events.
they're starters because you start with them.
they're shinies because they shine.