https://www.polygon.com/pokemon-go/2018/4/2/17188604/pokemon-go-where-to-find-latias-latios

Two beloved legendary Pokémon can be found in Pokémon Go starting today, April 2. Latias and Latios, the complementary dragon- and psychic-type monsters from the Hoenn region, are posting up at gyms worldwide and for a limited time.

The two Pokémon are known to travel in different directions and on their own paths, and Pokémon Go honors that by making the pair available in separate regions. Players in Europe and Asia will be able to fight Latias, the smaller of the two, in Raid Battles, while North American, South American and African players can try to capture Latios. After May 8, they’ll swap regions, giving everybody a chance to complete the set.