Are there Legendary Pokémon available now, in the wild?

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.