These Pokemon can be obtained through Pokemon Sun & Moon's QR scanner. Not only do they add the Pokemon to your Pokedex, but also unlock a trigger for you to catch them within the game. See each Pokemon's description for instructions on how to obtain them.
There is no rush to obtain these Pokemon as they can be obtained at any time providing you have met the in-game requirements.
Pikachu (Partner Cap)
Pokemon Ultra Sun & Pokemon Ultra Moon Only!
After scanning the QR Code below. Head to Pikachu Valley, located on Akala Island, Route 4. Talk to the man wearing a blue striped shirt standing next to the trailer.
The man will give you the Partner Cap Pikachu (Ash Pikachu) providing you have space in your party.
Don't forget to check out the top right area of Pikachu Valley with the Partner Cap Pikachu on your team, as there is a Pikachu you can talk to who will give you a Pikashunium Z. A Z-crystal only Pikachu can use!