https://www.polygon.com/2017/11/2/16597912/pokemon-ultra-sun-and-ultra-moon-version-exclusive-legendary-pokemon

Pokémon Ultra Sun and Ultra Moon are looking to be shockingly comprehensive, nostalgia-filled entries in the series. The latest update from the upcoming Nintendo 3DS games reveals that they’re loaded with legendary Pokémon — in fact, all legendary Pokémon from across the series will be obtainable.

From Pokémon Red and Blue’s legendary bird trio and Mewtwo all the way to Pokémon Sun and Moon’s legendaries, like Solgaleo, Lunala and Necrozma, players will be able to discover and catch them all in Ultra Sun and Ultra Moon. But they’ll need to sync up with someone who has the title they don’t own in order to collect the whole set.