In the anime, Ash’s first captured Pokémon after Pikachu is Caterpie. Pikachu was given to Ash by Professor Oak in the opening episode, while Caterpie became Ash’s first successful capture in the third episode, “Ash Catches a Pokémon.”
Ash encounters Caterpie in Viridian Forest and weakens it before throwing a Poké Ball. Although Misty is frightened of Bug-type Pokémon, Ash quickly becomes attached to Caterpie and is proud of catching it. The Pokémon proves useful soon afterward, using String Shot against Team Rocket and later evolving into Metapod.
Caterpie eventually evolves again into Butterfree, completing one of the anime’s earliest evolution stories. Ash later releases Butterfree so it can stay with a pink Butterfree it has befriended. This is sometimes confused with Ash’s first Pokémon overall, Pikachu, or with his first capture in a particular region. But counting the original anime journey, Caterpie is the canonical answer.