I don't know jap and i am not sure about the story of DP, but from how they design the legendary in the past, they usually group them together. from the first gen, 3 birds, mew & mewtwo, second gen, 3 legendary cats, 2 legendary birds, ho-oh & lugia, then the 2nd gen phantom pokemon celebi, to third gen phantom pokemon jirachi, and the 3 regis, latios & latias, and then the sea, ground and sky ruler pokemon, kyogre, groudon, rayquaza, and the sort-of third gen mewtwo, deoxys (one of the strongest) and then for the 4th, there are the 3 dragons, 3 physics, psychic and dark, and the creation pokemon (again the strongest of this gen), and finally left with manaphy, sheimi, and hiidoran, which is quite independent and not related to either, of course one of this, most prob manaphy is the phantom pokemon, but it could be sheimi too, and that leaves hiidoran. Oh as for regigigas, no doubt it is related to last gen, and which leads me to guess that hiidoran will have its counterpart in future gen? just a speculation , any idea?