Moved to Gaming and Technology
(it was a tossup between here and general discussion, but i guess seeing as this is about games)

I would consider Lunala and Solgaleo as both legendaries and Ultra Beasts.

I mean, think about it. To be classified as a legendary, you have to at least be considered very powerful and/or very rare. Both Lunala and Solgaleo are powerful and rare, and the Ultra Beasts are also powerful and rare. So I would consider both.

But then you have to take into consideration the Beast Balls. Beast Balls work very well catching Ultra Beasts, but are generally trash on ordinary Pokemon.
While it is impossible to catch the two legendaries in Beast Balls, since catching the main legendary isn't in the post game (when Beast Balls become available), and getting the Cosmog only automatically puts it in a regular Pokeball, with hacking, you can attempt to catch Lunala/Sogaleo with them. But people found out the Beast Balls don't get the boost, and just act as regular Pokeballs.
Going off that is pointless, since it's obvious Game Freak never took into account using Beast Balls on the two legendaries, but in a very technical level, according to the game, they are not Ultra Beasts.......technically.

As a Pokemon Sun player, I doubt that they aren't Ultra Beasts. Marked by the Pokedex, they're legendaries. But they do come from the Ultra Wormhole. And, in the Post-Game, summon Cosmog. But that's another story...

They're also the first legendaries to be part of an evolutionary line.

Well... I don't think ultra beasts were created back then.

What if, now that ive been thinking, that every pokemon that can travel through wormholes or portals (Dialga, Giratina, Ect.) Are technically ultra beasts?

Well, I like to go by the way that the Pokedex lists things. It tells you when you catch them if they're legendary or an Ultra Beast. Cosmog, Cosmoem and Solgaleo and Lunala and marked as legendaries, whilst Nihilego, Buzzwole, Pheromosa, Xurkitree, Celesteela, Kartana, and Guzzlord are marked as Ultra Beasts.

Keep in mind that it was thought that Necrozma was first thought to be an Ultra Beast, but that idea was dismissed by characters in the game and is backed up by the fact that it is also marked as "legendary" in the Pokedex instead of "Ultra Beast" (even though Necrozma also comes from another world).

Well both. Legendary Pokémon are just overrated Pokémon, mascots, important to the game's story, lore and legends. legendaries aren't a special thing, but Ultra Beasts have to be from the wormhole so Solgaleo and Lunala are Ultra Beasts and legendaries. As a fact, Solgaleo and Lunala are Ultra Beasts.