The latest edition of Japanese magazine CoroCoro has begun to leak and brought with it the reveal of Rockruff's Evolution as well as some more Ultra Beasts.
First up we have new Pokémon known as Rugarugan, which is the evolved form of Rockruff. It appears that Rugarugan has the ability to turn into two forms, one being the Midday form (pictured right) and the other being the Midnight form (pictured left). It's currently unclear if the Pokémon will remain in the form it evolves into or if it'll change based on the time of day.
Update: Thanks to some new pages of CoroCoro it has been confirmed that the two different forms of Rugarugan are version exclusive. Rockruff will only evolved into the Midnight Form in Pokémon Moon and the Midday Form in Pokémon Sun. The scans also reveal that the Midday Form has a new priority move called AccelRock, while the Midnight Form is shown off with Counter.