>>31494194How aren't they legendary? Only one of each can be found per-game, and almost all of the Trios are directly related to a big-league legendary, either as protectors/guardians or in some cases it is implied that the big-league legend created them.
Regardless, the Sun/Moon Pokedex has made a distinction that will help unblur the line for us, moreso once Pokebank/National Dex is out.
As of now, the Dex uses a different colored background depending on the category of Pokemon.
Big-league legends that being the Cosmog-line, Zygarde, Magearna and Necrozma have Gold/Yellow radiant backgrounds and a many large sparkling particle effects around their name.
Sub-legendaries, still legendary, but are allowed in post-game battle facilities, such as Type: Null, the Tapus and the Ultra Beasts all have a green background, and a smaller sparkling particle effect around their names.
All other Pokemon have Blue backgrounds and no sparkles.
Thus, we can make the assumption that Tapus/UBs are likely on the same tier of "legendary" as the previous Trios/Quads, and therefore the older Trios/Quads will likely also have the green background/name sparkles in the National Dex.
