In Short: Niantic has announced that Pokemon from Sinnoh, originally seen in Pokemon Diamond, Pearl and Platinum on the Nintendo DS, have invaded Pokemon GO, and it seems that they're scattered all around the globe. Niantic's announcement only mentions the three starter Pokemon for the generation by name, but the attached video shows many more, including legendary Pokemon Giratina, known in its game for control over dimensions. Some of the new Pokemon have already been found to be exclusive to certain regions of the world. Carnivine can only be found in the southeastern United States, Chatot hangs out in Australia and Brazil, and Pachirisu is mainly found in Canada, with limited sightings reported in Alaska.
Background: This rollout is likely going to be handled similarly to new-generation rollouts in the past. When the game first unveiled Johto Pokemon, they came out in waves, with some introduced in special events that made them available worldwide no matter where their usual home was. It was the same when third-generation Pokemon hit the scene. Legendary Pokemon have been handled a bit differently and somewhat out of order; Deoxys, a third-generation legendary, for example, came out before Celebi, a second-generation legendary. It should be noted that legendary Pokemon with significance or heavy mythology in the Pokemon universe, such as Mew, Mewtwo, and Celebi, seem to come out a bit later, and often out of generational order. Many Pokemon, particularly stronger ones, are exclusive to special events and raids even though they're not legendary. Absol, a very powerful and strategically versatile Pokemon from the third generation, is one of the most obvious examples; it seems to only appear in raid battles. Given the random nature of Pokemon that hatch from eggs, it's quite possible that some of these exclusive Pokemon could be found in eggs, as was the case with the likes of Chansey and Kangaskhan in the first generation. It's worth noting that there have not been any verified reports of legendary Pokemon coming from eggs.
Impact: Players can expect to see generation four starters appearing in the game right now, along with any region-exclusive Pokemon that may apply to them. The rollout will be happening in waves, so it will be a while, perhaps even a few months, before all of the fourth generation's Pokemon become available in the game. Niantic has yet to officially state what Pokemon will appear and when, or where for that matter. In fact, Niantic has not even officially confirmed that any Pokemon are region-exclusive. As for legendary Pokemon, since Giratina appears in the trailer video, it will almost certainly be appearing in a major event at some point in the near future. It's part of the Sinnoh creation trio, accompanied by Dialga and Palkia. This generation is also home to the God of the Pokemon universe, Arceus. Shaymin, a multi-forme Pokemon that appeared in a movie centered around Giratina will also likely be heavily featured, along with Darkrai, a legendary Dark Pokemon who was briefly the face of premium booster packs for the Pokemon card game.