Niantic has announced that Entei, Suicune, and Raikou, the three legendary dogs available in the second generation Pokemon handheld games, will be appearing in Pokemon GO very soon, and will be bringing field tests of a new EX Raid Battle feature with them. The new EX Raid Battle feature will be rolled out in select areas and only to select players initially, who will be able to access EX Raid Battles that pop up at random gyms and feature powerful Pokemon. This feature will roll out on a wider basis shortly after, granting EX Raid Battle passes to trainers who have completed a Raid Battle at that gym recently.
The three legendary dogs will be a bit more elusive than other legendaries up until now, in a way that pays homage to the different versions of each Pokemon game generation over the years. When the trio hits the streets on August 31, they will each be in different locations, then shuffle those locations on a monthly basis. Once all three have each been available for a month in all three regions, they will disappear until the next time Niantic rolls them out. Raikou will start out in the Americas, while Entei will first appear in Europe and Africa, and Suicune will pop up in the Asia-Pacific region. The first shift will happen at the end of September, then there will be another at the end of October. After October is over, all three Pokemon will have visited all three regions, and they will disappear again.
Niantic will be keeping trainers up to date on the progress of both the EX Raid Battle field testing and the movements of the three legendary dogs via social media. You can find official Pokemon GO accounts to follow for updates on both Twitter and Facebook. The EX Raid Battle field test will be dependent upon qualifying trainers who receive passes participating in EX Raid Battles, then giving Niantic feedback on how well they work, so there's no telling just how long that part of the new feature set will take. As for the legendary dogs, watch for the one you want to appear in your region and make sure not to miss your window, or windows, if you aim to catch more than one of them. As yet, there is no word on fellow second generation legendaries Ho-Oh and Celebi.