Pokémon Season 1 Episode 25

As Ash is on his way towards Celadon City, he encounters a Mankey. The Mankey starts chasing him and steals Ash’s hat. Ash tries to get it back and fails. Team Rocket arrives to get Pikachu and when Mankey walks by, James kicks it away and in its anger, evolves into Primeape. After remembering the poem Professor Oak said, Ash captures it.