Last weeks the Legendary Birds and Lugia were released. After an intense raid battle you could catch these beasts and add them to your Pokedex. However, you were not allowed to leave them in gyms, because they do not want to leave your side (but you can send them back to the Professor, strange enough).
So, if you do not had the opportunity to fight and catch one of these Pokemon, you have (right now, Niantic did not release more information) no chance to encounter them and add them to your Pokedex. The only way to see one of those is fighting alongside them in a gym or raid battle.
If you fight alongside one of these animals with an empty index, will it add it to your Pokedex (as seen, not as caught)?