"-Mew (Obtainable via the Japan-only Faraway Island Event in Pokemon Emerald)
-Celebi (Obtainable via the Ilex Forest event in the Virtual Console version of Pokemon Crystal)
-Jirachi (Obtainable via the Pokemon Colosseum/Pokemon Channel Gamecube Bonus Disc and a Japan/China only Gen VI Download event)
-Deoxys (Obtainable via the Birth Island Event in Pokemon FireRed, LeafGreen, and Emerald)
-Manaphy (Obtainable via Manaphy Egg, trading said Egg to a game other than the one it was downloaded in initially, and hoping it hatches into a Shiny)
-Darkrai (Obtainable via the Newmoon Island event in Pokemon Platinum)
-Shaymin (Obtainable via the Flower Paradise event in Pokemon Platinum)
-Arceus (Obtainable via a Japan-only Gen VI Download event)
-Genesect (Obtainable via a Japan-only Gen V Download event)
-Diancie (Obtainable via a Japan-only Gen VI Download event)"
https://gamefaqs.gamespot.com/boards/210930-pokemon-ultra-sun/77705529
Shiny Zeraora and Shiny Celebi are also available in Gen 8. (Shiny Zeraora was obtainable if one million people beat Dynamaxed Zeraoras, and Shiny Celebi is available from prebooking the distrubution of Pokemon The Movie: Coco)
Also, all event Pokemon are here
https://www.serebii.net/events/