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>Kanto: Kabuto and Omastar can only be revived by the 7th gym despite you getting a fossil after the 2nd.
>Johto: Never available at all in GSC, but in HGSS you get them as early as the Ruins of Alph... Only to have to wait until fucking Kanto to revive them.
>Hoenn: Not available in RSE. In ORAS, you have to wait until after beating Groudon/Kyogre.
>Sinnoh: Magically don't appear in the Underground until you get the national dex.
>Unova: NPC who hands out other region fossils is national dex exclusive. Apparently there might have been something with Join Avenue, but I think it pretty much died when they pulled the plug on wi-fi.
>Kalos: Magically don't appear in Glittering Cave until the national dex.