Bart's Castform is a normal-type Pokémon owned by Bart.
During a snowstorm, Castform greeted the heroes, who were approached by Bart and Millie. Castform went with Bart to the Weather Research Institute, which, later on, got attacked by Team Aqua members. One of the grunts summoned rain, causing Castform to change form and laid down, as a lightning struck Team Rocket, who were also captured. Bart placed his access card on Castform to protect info about the Ancient Pokémon from Team Aqua. Castform used Weather Ball on Team Aqua's Crawdaunt, who retaliated with Bubble Beam. The sun shined, causing Castform to change into another form and use Weather Ball, which hit the Crawdaunt and allowed Castform to float away. Castform came to the heroes, who protected it. As a part of the plan, Millie's Ditto transformed itself as a Castform to distract Team Aqua. Once Millie downloaded the data from the central computer, Ash bumped into them with the decoy Castform. Castform came out, where it encountered Team Aqua.