Castform

Castform is a -type Pokémon.

While it is not known to evolve into or from any other Pokémon, Castform has the ability to change forms depending on the current weather.
 * During normal weather, fog, a sandstorm, or a shadowy aura, it is in Normal form and is a Normal type.
 * During harsh sunlight, it is in Sunny Form and is a Fire type.
 * During rain, it is in Rainy Form and is a Water type.
 * During hail, it is in Snowy Form and is an Ice type.

Along with Pincurchin, Shuckle and Pyukumuku, it is one of the four starter Pokémon of the Mishi region.

Biology
Castform is an artificial Pokémon that changes appearance and attitude based on the weather. Its body is made of cells exactly like those of water molecules, causing its cellular structure to be chemically altered by temperature and humidity. Though not of its free will, Castform uses this ability to protect its tiny body. In its Normal Form, it resembles a plain, gray cloud with a single wisp extending from the top of its head.

Under bright sunlight, it takes on its Sunny Form. Its lower half becomes pure white, while the head turns orange has develops small orange spheres around it. This gives it the overall appearance of the sun above a small cloud. Its skin glows and dries, and becomes hot to the touch.

When it is soaked in rain, it takes on its Rain Form. It has a dark gray lower half resembling a storm cloud. The head turns blue and resembles a raindrop. Its body is soft and slippery and seeps with water when compressed.

When it is hit by hail, it takes on its Snowy Form. Its head turns purplish-blue. The rest of the body changes into the shape of a mint green, cumulonimbus cloud. Its body becomes a smooth ice-like material, partially frozen measuring temperatures near 23 degrees Fahrenheit.

Regardless of the form it takes, it will always have a large, spherical head and a lighter colored mask-like marking over its eyes. Various experiments were conducted to change its form in artificial conditions, but all have failed. Aside from its various forms, Castform is able to utilize weather powers in the form of its previously signature move, Weather Ball. Despite its man-made origins, it can be found in grassland areas.

Pokédex entries

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Trivia

 * Castform's Weather Ball will always receive STAB in any of its forms unless in a sandstorm, under Shadow Sky, or if it loses Forecast.
 * Castform is the only Pokémon to receive this benefit from Weather Ball.
 * Castform is the lightest Normal-type Pokémon.

Design origin
Castform resembles a living cloud and acts like weather in general, but it may also be based on a weather balloon. It is similar in both concept and design to a teru teru bōzu, a Japanese charm doll used to wish for good weather. Its forms and their types may have originated from the English saying "rain, hail, or shine". As Castform is said to be composed of molecules similar to water, its shape of a large sphere with two smaller orbs resembles an H2O molecule.

Name origin
Castform may be a combination of forecast (the prediction of weather) and transform or form (referring to its ability to change form depending on the weather).