https://www.gamespot.com/articles/pokemon-go-legendary-dogs-now-available-in-final-r/1100-6454548/

Pokemon Go players now have a chance to catch a new Legendary Pokemon. The three Legendary dogs from Pokemon Gold and Silver--Entei, Raikou, and Suicune--are each available in a new region for a limited time.

Unlike most of the previous Legendaries in Pokemon Go, each of the three Pokemon will only be available in a specific region for the duration of the month. This time, the Water-type Suicune can be found in the Americas; the Electric-type Raikou has moved to Europe and Africa; and players in the Asia-Pacific region can catch the Fire-type Entei.