Frosty Forest

Final Floor: 13F
Unlock Method: Beat Mt. Blaze
Initial Level: -
Maximum Teammates: 3
Items: Yes
Check Map: Yes
Opponents Able to Join: Yes

You will first encounter Articuno as you go through the Freezey Forest when a Fugitive from Pokémon Square. In this encounter you won't be able to befriend it.

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