https://www.gamespot.com/articles/pokemon-go-legendary-dogs-now-available-in-new-reg/1100-6453712/

A new Legendary Pokemon is now available in Pokemon Go. The three Legendary dogs Entei, Raikou, and Suicune have rotated to a different part of the world, giving players a chance to add another one of the powerful Pokemon to their collections before they make their final stop next month.

As before, each of the three Legendary Pokemon will only be available in their current region for the duration of the month. This time, players in the Americas will have a chance to catch the Fire-type Entei; the Water-type Suicune has moved to Europe and Africa; and the Electric-type Raikou can be found in the Asia-Pacific region. The three Pokemon will remain in their current location until October 31; on that date, the Legendaries will rotate to their final region, where they'll be available until November 30.