The Legendary Birds
When you have defeated the Elite 4, one of the three legendary birds from the original Pokemon Red and Blue will start roaming throughout Kalos. The bird which you will get depends on your starter (Froakie for Moltres, Chespin for Articuno, and Fennekin for Zapdos). Check the birds entry in your Coastal Kalos Pokedex after you have encountered one of the birds in the wild and it will show the bird's current location. If you walk (not fly) to that location and challenge the bird several times it will eventually retreat to Sea Spirit's Den giving you the chance to catch it.
Added by: Sanzano Oct 21st 2013, ID#870