Pokemon not native to the region they were first featured in; i.e. Safari Zone Pokemon, which are marketed as exotic and from other regions, rare migratory species, or simply Pokemon that cannot be obtained in their home region outside of a trade/in-game event.

I won’t count Pokemon that can be found in their home region in later incarnations of said region, such as Mr. Mime appearing in Kanto in GS/HGSS, because it’s obviously not a native and was introduced into that environment later either due to importation or as an invasive species

>all starters
Obviously
>all fossils
Possibly same region, but different time period