EDIT: I removed Deoxys and Mew from the list and added Type: Null and Silvally as legendaries. Thank you to Kempo for the corrections! I have recently created a Mythical Pokémon quiz.
Technically Deoxys is a legendary in my opinion because he is obtainable in the story for Omega Ruby and Alpha Sapphire, unlike other Pokemon such as Celebi which are only obtainable in special events. I will however allow the Tapus and Necrozma to be legendary. I'll make a separate quiz for the Mythic Beasts that are in the after story from sun and moon. I don't think Cosmog and Cosmoem are legendaries because i don't think they're obtainable through the story. Correct me if I'm wrong =)
I also don't think that Silvally is a legendary because it is a normal pokemon used by Lillie's brother in the story mode. I haven't played in a couple months so again correct me if I'm wrong. =)
You _can_ obtain another Cosmog (e.g. not Nebby) in the post-game of Sun/Moon, making it extra content in the same vein as the legendary trios of the past. And Cosmog evolves into Solgaleo/Lunala, which are undisputably legendaries
Bulbapedia considers Silvally to be legendary and Deoxys to be mythical (despite his inclusion in the Delta Episode). https://bulbapedia.bulbagarden.net/wiki/Legendary_Pok%C3%A9mon#Generation_VII
Silvally is far from a normal Pokemon though: it can't breed, only one is obtainable and it has a special Pokedex background (the same as the Tapus). But most importantly, the game treats it as a legendary: when you exclude legendaries from the list of deposited pokemon in the GTS, Silvally is excluded as well.
And personally, I am of the opinion that Ultra Beasts are legendaries as well, but their legendary status is still disputed so I'm not campaigning for their inclusion here.
Three years late but I took your advice and added the corrections. I don't know why I thought Deoxys was legendary when it clearly states he is mythical. Thank you for the corrections!
Could you provide me with the names of all the legendaries from Sun and Moon and up? I haven't played the game in two ears. If you can give me the names I'll add them in. Mythic Beasts do not count.
Not sure what I was thinking when I wrote this comment, seeing as most of the legendaries are there. Although Type: Null is missing. Also, there’s a random eighth column with no answers on the right.
Bulbapedia considers Silvally to be legendary and Deoxys to be mythical (despite his inclusion in the Delta Episode). https://bulbapedia.bulbagarden.net/wiki/Legendary_Pok%C3%A9mon#Generation_VII
https://bulbapedia.bulbagarden.net/wiki/Mythical_Pok%C3%A9mon#Deoxys
Silvally is far from a normal Pokemon though: it can't breed, only one is obtainable and it has a special Pokedex background (the same as the Tapus). But most importantly, the game treats it as a legendary: when you exclude legendaries from the list of deposited pokemon in the GTS, Silvally is excluded as well.
And personally, I am of the opinion that Ultra Beasts are legendaries as well, but their legendary status is still disputed so I'm not campaigning for their inclusion here.