Pokemon Sun and Moon: The Series has been running in Japan for almost three months, but Ash's actual Pokemon team has been rather thin. Ash picked up Rowlet early in the series, but he hasn't really encountered any new Pokemon in the wild since. It turns out that Ash didn't need to meet any wild Pokemon, he just had to befriend a Pokemon belonging to one of his friends.
In this week's episode, Ash discovered that Professor Kukui's Rockruff was disappearing at night and coming home injured in the morning. After following Rockruff, Ash and Kukui discovered that Rockruff was fighting (and losing) in an underground Pokemon fight club. Instead of discouraging Rockruff, Ash helped Rockruff train, which resulted in Rockruff learning a new move: Rock Throw.
Sure enough, Ash's training paid off as it beat a Magmar it had previously lost to the following evening. Seeing the sudden bond between Ash and Rockruff, Kukui released Rockruff so it could travel with Ash on his upcoming journeys through the Alola Region.
While it's a bit strange for Ash to take one of his friend's Pokemon, regional professionals do traditionally give Pokemon away to trainers all the time. Plus, Ash is still living with Kukui, so it's not like the hunky Pokemon prof won't get to see his old pal all the time. Next week's episode features the return of a stray Litten that has encountered Ash several times in the past, so it's possible Ash's team will grow again in the near future.