Is Solgaleo a must-have to complete the Pokedex in Moon?

Question

If you own the copy of, let's say Moon, must you also catch the other legendary and Ultra Beasts only found in Sun in order to complete your National Pokedex?

Answer 1

Yes, you must catch the Pokemon exclusive to the opposing game in order to complete both the Alola and National dex.

Source
Experience

Comments

there is no national dex in alola, thogh

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Well what they meant was that all Pokemon no matter if it's exclusive need to be seen and registered in the dex for you to be able to get the shiny charm (I'm assuming that's the point because well why else would you want to complete the dex).  Only Mythical Pokemons do not need to be caught to be completed and in the case of Alolan dex, it's Marshadow and Magearna.
EDIT:INCLUDING Zygarde because it counts in the Alolan Dex cause UB's attack, ecosystem threatened, Zygarde appears, you get the idea.