I want to know because shiny zeraora is awesome and I missed out on it so I want to know all of them.
-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)"
Source
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. These events have ended.)
Also, all event Pokemon are here.