New monthly events have been announced for the rest of this year, giving out legendary Pokémon! Each month a new pair of Pokémon is available, which appear to be the version exclusives that aren't available in that particular game (e.g. Dialga is available natively in Ultra Sun but by event in Ultra Moon).
The full catalog of Pokémon available during 2018
The full list of Pokémon for the year is below. Details for Dialga and Palkia have been announced - they are obtainable via a code from GameStop stores (USA), EB Games (Canada), GAME (UK) or your usual large game retailer in your country. Dialga is only available in Pokémon Moon or Ultra Moon, while Palkia is only available in Pokémon Sun or Ultra Sun. We'll provide details for the other legendaries as soon as they're available.