https://www.gamespot.com/articles/two-of-pokemon-gos-gen-3-region-exclusive-pokemon-/1100-6455925/

Pokemon Go developer Niantic recently expanded the game's Pokedex with the introduction of Gen 3 Pokemon, which arrived early last month as part of a big Pokemon Go update. As was the case with certain previous Pokemon, some of these monsters could only be encountered in a specific part of the world, namely Zangoose and Seviper, which were exclusive to the Americas/Africa and Europe/Asia, respectively. However, it appears the two Pokemon have now traded regions.

On Reddit, Pokemon Go players are reporting that they've begun encountering Seviper in America/Africa, while Zangoose is now appearing in Europe/Asia. Each creature's Pokedex entry has noted the longstanding rivalry between the two species, which makes this swap an amusing nod to the series' history. Niantic hasn't said whether or not the two Pokemon have traded regions permanently or if they'll return to their original locations, so players will want to catch them soon while they still have a chance.