https://www.gamespot.com/articles/last-day-to-catch-pokemon-gos-current-legendary-do/1100-6453592/

Pokemon Go players only have a few more hours to catch Entei, Raikou, or Suicune. The three Legendary dogs from Pokemon Gold and Silver will only be available in their current region through September 29, after which they'll move on to a different part of the world.

Unlike the previous Legendary Pokemon that have appeared in Pokemon Go, only one of the three Legendary dogs is available in a specific region at a time. Currently, the Electric-type Raikou can be found in the Americas; the Fire-type Entei is available in Europe and Africa; and players in the Asia-Pacific region can encounter the Water-type Suicune. On September 30, the Pokemon will rotate to a different region, where they'll be available until October 31.