Or perhaps a third legendary that rules over both of them. I'm not entirely sure how that would work, given we have Pokemon that each symbolize the Earth, Moon, and Sun. Perhaps a Pokemon with darker connotations?
Now I'm wondering if Tapu Koko is even a Pokemon. Ancestor worship was a thing in Hawaiian religion after all, so Hala could've been talking about the ancestors of the Alolan people.
That would be intimidating. I too imagined the Souvenir to be a Giant Pokémon as big, or bigger than Regigigas. I do see one emerging from the lava of a volcano. Another from the ocean. One from a forest, and another from sand or stone. So a Fire, Water, Grass or Flying, or both, and Ground or Rock, or both Quad. Funny enough the starters and Iwanko have all those types, except for Ground (though it's rumored that Litten or Rowlet's final could be part ground).
Something like Shadow of the Colossus, except with Pokémon? I suppose you could have that, but who's going to kill Lillie to start the plot?
You could have them be legendaries of 'special,' or set-off islands that can't usually be entered, but generally speaking it does seem unlikely that these two would otherwise just be two of the four legendaries there.
