>>26913342.
In the E3 gameplay, it is mentioned that each of the 4 island has a guardian deity
Tapu Koko is the 'first' islands deity
They have a festival in it's honor and a pokémon battle
Hawaiian myths say that Hawaii has 4 'main' gods, Kāne, Kū, Lono and Kanaloa (ka hā/akua).
Ku is the deity of war and in his rituals there has been human sacrifice
Tapu Koko supposedly means something like Holy Blood
This and that Tapu Kokos celebration includes a pokémon battle, makes me think Tapu Koko originates from Ku, the deity of war.
(* Ku was also Husband to Hina, goddess of the Moon)
(* Ku's counterpart would be Lono, the god of peace. It's festival requires that there be no war or unneccessary work during it's period)
(* Then we have Kane and Kanaloa closely linked with eachother. Kane being the giver of life, associated with dawn, sun and sky. While Kanaloa was the god of the underworld. Also symbolised as a squid or octopus.)
(* It is said that either all except Kanaloa, created humans by mixing clay from 4 different corners. Or that Kane did it himself and that Kanaloa tried but failed so he made humans have a finitie life span, so that he could claim them after death)
(* It's also said that in the beginning there was only night/darkness and that Kane pulled himself free from it and the others followed suit and created light)
This is just a wacky theory and many questions still need to be answered to take this serious. Like how would Lunala and Solgaleo fit in this? Is Tapu Koko a pokémon? How come the protagonist already has met Tapu Koko and was it meant in a literal sense?