Are there Legendary Pokémon available now, in the wild?

Disclaimer: this information is not official, as no information has been officially released regarding catching legendaries.

There are a total of five Pokemon we can get from Niantic’s augmented reality game: the Legendary bird trio Zapdos, Moltres and Articuno; the Mew Duo, Mew and MewTwo. Based on the first trailers of Pokemon GO, these powerful Pokemon might be acquired through events that can be participated in by anyone. If this is the case, all we have to do now is wait for the events themselves.

If Niantic will follow suit from the event pattern of other Pokemon titles, we may have a chance to catch a Legendary monthly.

If Niantic will choose a different path, it may release Legendary events seldomly to stay true to the franchise by letting Legendaries be a rare sight.

Another possibility is that Niantic may release the first set of Legendaries when Pokemon Sun and Moon launches on Nov. 18.

What we can expect though is that Niantic won’t let Legendary Pokemon be exclusive to certain areas since it’ll be unfair to those who don’t have the capacity to travel in far-off places. -- source

The team logos are the legendary birds, so there may also be a connection between your chosen team and the legendary bird(s) you can catch.

In the two years since this question was posted, we now have confirmation as to the how and when distribution of legendary Pokemon occurs. There are three primary distribution methods:

• Raids - Raids pop up on Gyms, and battling in a raid pits you and your friends against a powerful Pokemon with a chance to catch it at the end.
  • Raid Pokemon aren't always necessarily legendary Pokemon
  • Special raid times may be announced which will see all Gyms host one raid Boss in particular, usually a legendary.
  • Ex-Raids generally host the powerful legendaries such as Mewtwo and Deoxys.
• 7-day Field Research Rewards - every month a particular Pokemon will be the reward for the 7-day research tasks, including Minor/Trio legendaries (such as the Kanto birds, Johto dogs).
  • Research reward Pokemon aren't always necessarily 'legendary' Pokemon. They can also contain rarer Pokemon, like Snorlax and Shedinja.
• Special Research Quests - Mythical legendaries (Mew, Celebi) have been released as part of larger research tasks with multiple stages to complete before obtaining them.
  • Once completed, that's it - so far it seems everyone can only get one of these Pokemon.

Keep an eye on Community announcements in the 'News' section of the app, as a lot of the events and community days are announced there first in the weeks leading up to the event.