Castform is an artificial Pokémon created by the Weather Institute in Hoenn. It was designed to help scientists predict the weather, and consequently changes its appearance (and type) when the weather changes.

In its natural state, Castform is small, grey and resembles a cloud. When it is extremely sunny, Castform's lower half becomes pure white while the head turns orange with smaller orange balls surrounding it. This gives Castform's head a shape that is similar to the sun. During rain, Castform's lower half becomes a darker shade of gray. The head is blue, resembling a drop of rain. In hail or snowy weather, Castform's head turns purplish-blue. The rest of the body changes into a mint green cumulonimbus cloud that also appears to resemble a decorative ice sculpture. Castform does not have a form for sandstorms or fog.

In all forms, Castform has markings around its eyes similar to a pair of sunglasses.