http://www.techradar.com/news/gaming/pokemon-go-updates-everything-you-need-to-know-about-what-s-coming-next-1330140

Update: Legendary bird Pokemon, Articuno, Moltres and Zapdos, have now cycled out of Pokemon Go, but they're being replaced by the legendary dog Pokemon from the Johto region.

Which of the Pokemon you can catch depends on where in the world you live – electric-type Raikou is currently in the Americas, fire-type Entei is in Europe and Africa, while water-type Suicune can be caught in the Asia-Pacific region.