Stoutland was a friend of a wildLitten that wandered around Melemele Island. It was an elderly Pokémon that Litten looked out for, by giving it food, and keeping it company. Ash intended to catch Litten but he gave up when he discovered the Pokémon giving food and company to Stoutland.
In One Journey Ends, Another Begins..., Stoutland was visibly worse in condition and coughing constantly. It attempted to teach Litten Fire Fang by demonstrating it, but Litten ended up hurting itself when it attempted to bite a rock. Stoutland later collapsed, and Litten ran off to get Ash's help, who proceeded to carry it to a Pokémon Center. Stoutland was taken into the center's emergency room and hooked up to an electrocardiogram as it was attended to by the center's Nurse Joy and her Blissey and Comfey. Nurse Joy came out to the waiting room to inform Ash that Stoutland was not ill or injured, and Litten rushed through the open door to check on Stoutland while the two humans agreed that they knew what was going to happen.
Stoutland briefly reuniting with Torracat
Stoutland later left the Pokémon Center and returned to its home under the bridge with Litten, unsuccessfully attempting to teach it Fire Fang again and falling asleep during Litten's turn. After curling up into Stoutland afterwards, Litten awoke in fear after a dream where it was unable to keep up with Stoutland and discovered it had disappeared. Stoutland's disappearance and Litten's unsuccessful search for it coincided with a heavy rainstorm. After being found by Ash, Litten chased a tree's last leaf when it was blown away by the wind after the rain stopped. It then discovered a cloud formation shaped like Stoutland smiling down upon it.
In Memories in the Mist!, Stoutland appeared as a ghost inside Tapu Fini's mist, confirming that it had passed away during or after the events of One Journey Ends, Another Begins.... It was briefly reunited with Torracat, who proudly demonstrated its moves to its mentor. Stoutland then helped Torracat to upgrade its Ember into Fire Blast before the mist was lifted and the Big-Hearted Pokémon departed back into the afterlife.