>>58656294Regigigas is explicitly stated to be one of 3 "Creator Gods." Pokemon that create other pokemon outside of Arceus who produces all Eggs.
Lugia imagines the Mirage Trio from it's psychic powers
Ho-oh Reincarnates the legendary beasts from its revival powers
Regigigas can turn Earthly Material of any type into a "Giant," hence the legendary giants moniker the Regis all bear, and Regigigas being "King of the Giants."
Considering that the Galarian forms imply that Lugia has made more than 1 set of Mirages, and that the paradox forms of the legendary beasts imply Ho-oh has revived more pokemon than the 3 we know, it is safe to assume likewise that Regigigas has produced "giants" of every element as he towed the continents.