Ash captured Kingler as a Krabby so he could prove to Misty that it was not a fluke that he captured his other Pokémon. Immediately after the capture, Krabby's Poké Ball teleported away, due to Ash already having six Pokémon in his team. Brock later explained that Krabby was sent to Professor Oak.
Ash decided to use Krabby in the first round of the Pokémon League, reasoning that it would be perfect on a Water battle field, despite the fact that Krabby had never battled before. Krabby managed to defeat Mandi's Exeggutor and the experience it subsequently gained during the battle caused it to evolve into a Kingler.