Pokemon: Season 20, Episode 4

20 - 4: First Catch in Alola, Ketchum-Style!

After Satoshi fails to capture two wild Pokémon, he encounters a Mokuroh that lives in a Tsutsukera nest. After Satoshi feeds the starving Mokuroh, it returns the favor by helping him fend off Rocket-Dan away from its nest. Afterwards, Satoshi catches Mokuroh as his first Pokémon in Alola.