The official Pokémon movie website has updated. Translations follow:

The Legendary Pokémon Lugia Appears in the New Movie!

In the teaser for the new movie, we could see the shadow of a flying Pokémon reflected in the water surface ...
We will now disclose the true identity of this shadow!

Believe it or not, the legendary Pokémon Lugia will appear in the new movie!

The impressive melody that played in the second half of the teaser was the flute music that played
during important scenes in the second movie, Pocket Monsters the Movie: "The Explosive Birth of the Mythical Pokémon Lugia" from 1999.

Lugia has played a major role in past movies as Satoshi's ally. What role will it play in the new one?!
We will keep publishing follow-up news here on the official movie website, so stay tuned!

Click here to see the teaser

Lugia first appeared in Pocket Monsters the Movie: "The Explosive Birth of the Mythical Pokémon Lugia" (1999) under the moniker of the mysterious Pokémon "X", alongside a silhouette.
It was later announced that its picture would adorn the packaging for "Pocket Monsters Silver", which became the talk of the town.
Recently it has appeared in Pokémon XY the movie: "The Archdjinni of Rings: Hoopa" (2015), as well as in raid battles in "Pokémon GO".
In the "Pokémon Elections 720" held in 2016,
it held the honor of being the highest ranking Pokémon from "Pocket Monsters Gold/Silver", proving that it's a legendary Pokémon that's both well known and popular regardless of generation.
Click here for information on Pocket Monsters the Movie: "The Explosive Birth of the Mythical Pokémon Lugia"
Click here for information on Pokémon XY the movie: "The Archdjinni of Rings: Hoopa"





Source: http://www.pokemon-movie.jp/news/?p=3199