https://www.gamespot.com/articles/last-chance-to-catch-pokemon-go-legendary-dogs/1100-6455261/

If you've yet to catch Entei, Raikou, and Suicune in Pokemon Go, you only have a few more hours before they disappear. The Legendary trio will only be available in their final regions until tomorrow, November 30, making this your last chance to add the powerful monsters to your team.

The three Legendaries first began spawning in Pokemon Go back in September. However, unlike previous Legendary Pokemon, which were available globally for a limited time, each one was exclusive to a specific region of the world for a month; after that, they rotated to a different region. Suicune can currently be found in the Americas; Raikou is available in Europe and Africa; and players in the Asia-Pacific region can encounter Entei.