After you beat the Elite 4, one of the three legendary birds from the original Pokemon Red and Blue will start roaming throughout Kalos. Which bird you get depends on your starter. If you chose Froakie, you'll find Moltres. If you chose Chespin, you'll find Articuno. If you chose Fennekin, you'll find Zapdos. After encountering one of the birds in the wild, check its entry in your Coastal Kalos Pokedex. It'll show the bird's current location. You must walk - not fly - to the location and challenge the bird again. After fleeing several times, the bird will eventually retreat to Sea Spirit's Den, finally giving you the chance to catch it.