>>45402925It is a complicated subject. Even with some legendaries being theorically unique (Mewtwo or Arceus), there's multiverses where this unique Pokémon can exist (A.K.A. the savefiles for example), thus having more than one possible.
Even so, I have another theory. I think that Legendary Pokémon, when "catched", just allows you to get a copy, a clone of them, and never the real deal, A.K.A. one of Arceus' "1000 hands" if you will. There's some instances where this contradicts itself however (like the Gen V dragons and the orbs or recently Calyrex saying he himself wants to go with you), but that's the only explanation I can give to the unique cases. Well, except Mewtwo, that one is not logically possible really.
Even with that theory in mind, the vast majority of legendary Pokémon are more than just one of its species, like usually the majority of trios like Regis (well, trio no more), the Dogs or the Birds, Lugia / Ho-oh, Celebi, Latios / Latias, and some others. So yeah, I think shiny Legendary Pokémon should be a thing
