I just really wanna find zygarde...This is possible
"In both games, all legendary Pokemon will be found in Ultra Wormholes, which are found in the Ultra Space. Here are the exclusives for each version."
https://www.gameskinny.com/8iqfn/pokemon-ultra-sun-and-ultra-moon-legendary-exclusives
But if you notice, Deoxys is not there, and I doubt he's a now considered Mythical.
"Contrary to its classification in Pokémon Omega Ruby and Alpha Sapphire, PokémonCenter.com notably calls Deoxys a Legendary Pokémon rather than a Mythical Pokémon, creating some confusion over its official designation."
https://www.google.com/search?clien...+or+mythical&sourceid=opera&ie=UTF-8&oe=UTF-8
https://bulbapedia.bulbagarden.net/wiki/Mythical_Pokémon
There's also Mr. Chuuster with a reporter. Maybe when you fulfill enough events (wedding, Chuuster, certain 'mon, etc,), the Pika valley changes?