Pokemon Sun & Moon EP44 - A Dream Encounter!

After a strange dream involving the Legendary Pokémon Solgaleo and Lunala, Ash has a hard time waking up. On his way to school, he's sidetracked by Tapu Koko, who guides him to a little Pokémon that looks like a cloud of stars and that Rotom can't identify. Lillie nicknames it Nebby, and Ash remembers his dream: he had promised to take care of it! This attracts the attention of Lillie's mother, Lusamine, the head of the Aether Foundation. She and her colleagues come to meet Nebby and offer to care for (and research) this unknown Pokémon. But Ash is resolute in his promise, so Nebby's identity is a mystery for now.