Question

Are there any legendaries in sun and moon other than Solgaleo and Lunala? And if not could ultra beasts be replacing legendaries?

Comments

There are different classes of "legendary".  Type: Null and Silvaly are also legendaries, as are Magearna, Necrozma, and Marshadow.  Presumably, the Ultra Beasts are a new class of legendary.

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The whole definition of legendary has been screwed over this generation haha.

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TBH it has been for a few, not just now in Alola

Answer 1

A list of the Legendary Pokemon in Sun and Moon are:

• Cosmog & Cosmoem
• Island Guardians (Tapu Koko, Tapu Lele, Tapu Bulu, Tapu Fini)
• Necrozma
• Magearna
• Marshadow

And while Zygarde is available in Alola, it's still technically a Gen VI Legendary.
Furthermore, the Ultra Beasts are available although their Legendary status is disputed.

Hope I helped. :)
Source

Comments

Keep in mind that even though Serebii does not classify Type: Null and Silvally as legendaries, the games' code says they are. Some people still debate this though, so it is undecided

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Could you describe ultra beasts as pseudo-legendary?

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Nope, there is a specific criteria for "pseudo legendary" that Ultra Beasts don't fit. They are their own separate entity.

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magearna  and marshadow are mythical not legendary