Pokemon Go's newest Regional Pokemon were meant to be together, but are separated by two giant oceans.
Players have tentatively figured out that Plusle and Minun are the new regional-exclusive Pokemon. Plusle and Minun are "related" Pokemon, meaning that they have some sort of natural connection even if they don't evolve from one another. Plusle and Minun look almost identical, except that one has red ears and a plus sign-like tail and the other has blue ears and a tail shaped like a minus sign.
In almost every form of Pokemon media, Plusle and Minun appear together. In fact, Plusle has only appeared one time in the Pokemon anime series without a partner. At one point, Super Smash Bros Brawl even considered using both Plusle and Minun together as a fighting pair similar to the Ice Climbers.
However, Pokemon Go has cruelly split these two Pokemon apart on separate sides of the globe. Plusle appears in North and South America, while Minun appears in Asia, Australia, Africa, and Europe. It's unclear whether there's an exact demarcation line where the two Pokemon split off, or if it's simply determined by continents.
Of course, Pokemon Go fans should still be pretty pleased by the game's choice of regional-exclusive Pokemon this time around. Rather than picking out a useful Pokemon like Heracross, the game's developers instead picked two Pokemon that won't have any real impact on the metagame.
Right now, it appears that Plusle and Minun are the only regional-exclusive Pokemon added to the game. We're still sorting through all the Gen 3 Pokemon Go news, though, so stay tuned!