Articuno can be encountered in Seafoam Islands F1, Seafoam Islands F2, Seafoam Islands F3, Seafoam Islands F4, and Seafoam Islands F5, when your leading pokemon is alive and holding a Feather of Articuno. This feather can be dropped from any pokemon on the same aforementioned maps, when your leading pokemon is alive and an ice-type.
To confirm you have a chance at encountering one of these legendaries, type /pokemon and make sure they’re on the list under the “Legendary” category. The encounter rate of these pokemon is 1 in 200,000, and will always be encountered at lv 85.
Full credits to BRODY for the post