You can get Yveltal in Y and Xerneas and X just by beating it. After you beat it you can get Zygarde, one legendary bird, and Mewtwo.
Zygarde - the back of Terminous Cave
Moltres - Start with Froakie
Zapdos - Start with Fennekin
Articuno - Start with Chespin
"The way this is done is that after defeating the Elite Four you encounter your bird. This puts its entry into the Pokedex and enables you to track it using the location feature on the device.
The location given will only tell you of the route it is on. Once there, you need to wander around in the bushes hoping for a random encounter.
If you do manage to find it, prepare to be disappointed, because it’ll immediately run/fly away.
You’ll have to track it down again. This cycle will continue for 10 turns, before the flighty bird settles in the Sea Spirit’s Den where you can capture it in peace."
Mewtwo - Go to the Pokemon village, there will be a cave to surf to. Mewtwo is in that cave. In X it comes with Mewtwonite X and in Y it has Mewtwonite Y
Rotom is also a thing but not quite legendary. It's found in the Lost Hotel if you enter from route 16. You need strength and rock smash for it.
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You can get Yveltal in Y and Xerneas and X just by beating it. After you beat it you can get Zygarde, one legendary bird, and Mewtwo.
Zygarde - the back of Terminous Cave
Moltres - Start with Froakie
Zapdos - Start with Fennekin
Articuno - Start with Chespin
"The way this is done is that after defeating the Elite Four you encounter your bird. This puts its entry into the Pokedex and enables you to track it using the location feature on the device.
The location given will only tell you of the route it is on. Once there, you need to wander around in the bushes hoping for a random encounter.
If you do manage to find it, prepare to be disappointed, because it’ll immediately run/fly away.
You’ll have to track it down again. This cycle will continue for 10 turns, before the flighty bird settles in the Sea Spirit’s Den where you can capture it in peace."
Mewtwo - Go to the Pokemon village, there will be a cave to surf to. Mewtwo is in that cave. In X it comes with Mewtwonite X and in Y it has Mewtwonite Y
Rotom is also a thing but not quite legendary. It's found in the Lost Hotel if you enter from route 16. You need strength and rock smash for it.
You can catch most of the legendaries in Pokemon X but following the main story, You can only get Xerneas and Zygarde (Yveltal for Pokemon Y)