This is a long post so I am putting it under a cut. If I missed something, tell me.
Obviously, Ingo. (BRING HIM HOME, TPCI.)
Obviously, again, the Forces of Nature.
Also, Zorua and Zoroark, though how they got from wherever they are from to Hisui is a question. (Personally, I think they are natively from Galar/Isle of Armor, since even in Unova, they were a special event pokemon.)
I forgot about the Petilil line SOMEHOW despite it being one of my favourites. Not sure how this one got from Hisui to Unova, or vice versa (I think it went from Hisui to Unova, because it has to be around for the Ancient Hero to have a Lilligant.)
Less confusing about how they got to Hisui, the Rufflet line, the Oshawott line, and Basculin.
Alder and the suspiciously similar Diamond/Pearl Clan venerated member
Security Corps' secret weapon Ress and his impressive yet familiar sideburns. (Picture of Emmet to emphasize the modern continuity of the sideburns and to include him in this post. Ancestor possibility? I don't want to Futurama the twins.)
Other people who look Suspiciously Similar, such as Lian (to Clay), but I don't feel like adding another picture. We know what they look like.
Pokemon found in Unova (BW2) and Sinnoh/Hisui include the following (condensed list): Zorua line*, Rufflet line*, Oshawott line*, Basculin*, Petilil line*, the Forces of Nature, Riolu line, Azurill line, Magnemite line, Growlithe line*, Magby line, Elekid line, Zubat line, Onix line, Gligar line, Mantyke line, Remoraid line, Spheal line*, Swablu line, Bronzor line, Vulpix line*, Sneasel line, Swinub line, Ditto*, Lickitung line, Yanma line, Carnivine line, Tangela line, Nosepass line, and Croagunk line.
White Forest Pokemon not included in the base dex of Unova (BW2): Abra line, Machop line, Gastly line, Rhyhorn line, Porygon line, Togepi line, Wurple line, Ralts line, Shinx line, Budew line, Wooper line, Wingull line, and Happiny line.
Those with a star either appear in Unova and Sinnoh but not Hisui, or in Unova and Hisui but not Sinnoh.
I didn't include Dreamworld pokemon.
I just want to take a minute and point out that Warden Ingo's Hisuian team consists of four pokemon that are found in Unova: Gliscor, Tangrowth, Magnazone, and Probopass. One more (Abra) can be found in the White Forest. (SEND THIS MAN HOME.)
Arceus' plates can also be found in Unova, and while this may have just been a way for the devs to make sure that their data was available to players, they were placed in a significant location for the players to find in-game rather than being sold at auction, like in Paldea.
