>>44405647I have a hunch that Regigigas was actually planned for Gen III.
My reasoning for that is the recent Gigaleak where a whole bunch of development WIP sprites were uncovered, and a majority of legendary/mythical Pokémon had placeholder, incomplete, or temporary designs.
Regigigas had a crude rendition of its final design, which might be carried over from Gen III development.
https://youtu.be/vVzMytJ1w0c?t=169Pokémon being cut from a prior generation to be finalised in a following generation isn't unheard of either. I recall this being the case with Haxorus.
