Over the past couple of months, we have seen Generation III Legendaries Groudon, Kyogre and now Rayquaza each become available in Pokémon Go. Now, for a limited time, all 3 will be in the game at the same time!
Until March 5th, the signature Legendaries of Pokémon Ruby, Sapphire & Emerald can be fought in Gym Raid Battles. If the community defeats Rayquaza more than Groudon & Kyogre over this time, then Pokémon that like windy weather will be more likely to hatch from Eggs between March 5th-16th. Bagon, which evolves into the mighty Salamence, is one such Pokémon.
Alternatively, if Groudon & Kyogre are defeated more, then Pokémon that prefer sunny or rainy weather (for example, Trapinch and Lotad respectively) will have a higher probability of appearing from Eggs over that specified period. The community may be rooting for Rayquaza to be defeated more, as Bagon is hard to acquire outside of this event.
In addition, the in-game Pokémon Go shop will feature special item packs at discounted rates. Be aware, these packs will only be available for a finite amount of time.
This event comes off the back of Generation III Pokémon being gradually added to the game. The latest addition, which came with Rayquaza, brought in dragon- and flying-type Pokémon.