Pokémon go rumor spreads that legendary bird Pokémon Articuno, Zapdos and Moltres should appear in the new update. It is said that they should be able to get in Pokémon Go after collecting 3 feathers each one after an expedition told to you at a nearby pokegym. After collecting three feathers, which are said to appear rarely during your expeditions from pokestops, you would need to bring them to a nearby gym which is taken by your team (Mystic, Valor, and Instinct). After that, you would be challenged to a battle with the legendary Pokémon, and after a win, you would get a pokeball corresponding to your team with which you would catch the legendary Pokémon with 100% capture rate.
One thing is still unknown - will you get the pokeball back if you miss the throw? Or will you need to redo the whole feather collecting procedure again? Additionally, only one of the three legendary Pokémon would be available to each player, each one corresponding to your team: Zapdos for team Instinct, Moltres for team Valor, and finally Articuno for team Mystic. If you hear that some people already have one of the legendary bird Pokémon, it's not true, because they are not out in Pokémon Go yet.
Unfortunately, nothing is yet known about Mewtwo, Mew and Ditto the other legendary Pokémon from gen 1, except that they may be released in some event, or otherwise in some big update.
Articuno: Articuno is a legendary Ice and flying type Pokémon. As it flies, snow will fall because with every flap of its wings it freezes the air and makes beautiful snowflakes. It mainly lives atop snowy mountains where it is cold and isolated from other creatures.
Zapdos: Legendary Electric and flying Pokémon Zapdos lives in thunderclouds. During flight it causes crackling and snapping sounds of electricity as it discharges thunderbolts and causes thunderstorms. Every time it gets hit by lightning it grows stronger.
Moltres: Fire and flying legendary Pokémon Moltres is very rarely seen just like the other legendary bird Pokémon. Because it is covered in flames, when it flies, embers fall from its wings. Moltres has the ability to heal itself in molten magma from active volcanoes if it gets injured.