>>57838316Giratina overwrote a Pokemon tentatively titled Kimairan. Its pokedex entry was
>A Pokémon that lived a long time agowas born again. They are now extinct. They call out for others in their kind.
The three jewels in their body and its name (based off Chimera) imply that it was formed perhaps artificially. Back then, it was only Ia (Dialga), Ea (Palkia) and Aus (Arceus) being referred to together in all the myths. Perhaps because it would be too far to have Arceus be the third version, they made him a mythical and created a 3rd member in line with the Creation Trio's design cues with similar inspirations to Kimairan (basilisks)
This also explains why Giratina's final place in the pokedex is alongside the other random legendaries, because it overwrote Kimairan. The dev staff even admitted in interviews for Platinum they had no idea what Giratina was even meant to be until it came time to make the game.
Basically, Giratina was quickly tacked on to DP based vaguely off a unrelated legendary to round off the Creation Trio and had more iterated in it post-release. Kimairan last appears in the July 2006 build, which suggests Giratina was close to last minute to have even made it to DP and Battle Revolution.
