>>56626156We got versions of the Sinnoh creation myth from the leak. One version of the myth has it so that the fragments of Arceus' egg after it hatches turn into giants that immediately attack Arceus. Arceus defeats them and uses their bodies to create Dialga, Palkia, etc. People are theorizing based on them being called giants, as in gigas, and the Regis seeming connection to the plates, that have always been referenced as having the power of defeated giants in them, that Regigigas is one of those giants.
Bear in mind there's a footnote on one of the drafts that says the giants aren't pokemon, and there's also other versions where there's only one giant that gets dismembered to make the gods, and other versions where its implied to be giratina