Pokémon Season 9 Episode 34

On the way to Indigo Plateau for May’s Grand Festival, a group of Aipom come by and steal Ash’s hat and May’s ribbons! A old man comes by and tells them that local Aipom, Mankey, and Vigoroth have been stealing his berries and taking them up to the mountains. He also tells them that there is a rumor about a giant monster living in the mountains.