Pokémon: Slaking Kong

Animated series. Ash and Pikachu return to the Kanto region to complete the Battle Frontier. 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! An old man passing tells them that local Aipom, Mankey and Vigoroth have been stealing his berries. He than tells them about a rumour that a giant monster lives in the local mountains.