Pokemon Sun & Moon - episode 21 - 05 Apr, 2017

Ash and Pikachu run into Litten again, but this time something's very wrong with its friend, the old Stoutland. Ash carries Stoutland to the Pokémon Center, where Ash, Pikachu, and Rotom learn that while Stoutland isn't seriously ill, it is getting very old and there is nothing Nurse Joy can do to help it. Litten and Stoutland escape from the Pokémon Center and return to their riverside den, where Stoutland passes away in the night. After realizing what has happened, Litten is heart-broken and refuses to be comforted by anyone. After Litten finally comes to terms with the loss of its friend, Ash asks Litten to join him, and Litten decides to test him by battling against Pikachu. When the battle ends in a draw, Litten chooses to become one of Ash's Pokémon. Team Rocket's Meowth, who for reasons of his own has been secretly watching everything, seems pleased by the outcome. (Source: Wikipedia)