It should be.
Following are a few cases in which it won't though:
- If you encountered it in a trainer battle against another player
- If you encountered it in a Battle Facility (Battle Institute, Maison or Chateau)
- If you encountered it wild, then turned off your game without saving after that
The third option is highly possible, in which case you have to find it again. Either this or you have had misunderstanding and haven't met Articuno at all.
Hope I helped!