Dawn's Ambipom was caught by Ash but he traded her for Dawn's Buizel because Ambipom preferred contests over battles.
Ash caught Ambipom as an Aipom in the Battle Frontier arc, but it had appeared in a few previous episodes prior, causing Ash trouble by stealing his hat. This eventually provoked Ash into battling and catching Aipom during the Grand Festival. However Aipom still continued to steal Ash's hat whenever it seemed jealous of Pikachu's bond with Ash.
Aipom chose to travel with Ash when he went to Sinnoh, helping him defeat Roark and Gardenia. After being advised by their friend Zoey, Ash traded his Aipom to Dawn for her Buizel, as Aipom loved contests, while Buizel liked to win battles. It is the third Pokémon Ash ever traded. While in the Solaceon Ruins, Aipom evolved after using Double Hit in order to defend Dawn from the enraged Unown.