Mt. Thunder

Final Floor: 12F
Unlock Method: Complete Jumpluff's mission and find Zapdos
Initial Level: -
Maximum Teammates: 3
Items: Yes
Check Map: Yes
Opponents Able to Join: Yes

You will first encounter Zapdos as you go through the Mt. Thunder. In this encounter you won't be able to befriend it.

When you return to Mt. Thunder after saving the world in the postgame, you'll find Zapdos there. Defeat it and it'll be recruited and, if you haven't already got it yet, give you the Legendary Island Camp