Over the years, the Pokemon games have introduced an extensive number of creatures that could hit the thousand mark with the upcoming Pokemon Scarlet and Violet. Many of these Pokemon have been given further variety with several forms, ranging from the all too familiar regional variants first introduced in Pokemon Sun and Moon to much rarer visual distinctions like Sinistea's and Polteageist's phony and antique forms from Pokemon Sword and Shield.
One such Pokemon that arguably began the trend of multi-form creatures across future Pokemon games was Castform, the Weather Pokemon. Thanks to its unique "Forecast" ability, Castform would transform dependent on the weather in battle and take on a new type, receiving Sunny, Rainy, and Snowy forms respectively. But while the sandstorm effect existed at the time when Castform was introduced in Pokemon Ruby and Sapphire, Castform never received a Sandy form despite the ideal opportunity to round out all forms of weather. Since the weather continues to be a core mechanic in Pokemon, Game Freak should revisit this unique Pokemon and complete the Castform set.
Weather Ball Changed But Not Castform
The most significant evidence that Game Freak has acknowledged sandstorm as an important weather effect like rain or hail lies with the move Weather Ball. When Castform was introduced, Weather Ball was its signature move, and would not only change type alongside Castform with the weather, but would also double in power. This provided Castform with several weather-based forms and a unique gimmick that made it worthwhile addition to a team. For instance, during hail, Castform and Weather Ball would become Ice-types, and if Castform knew other weather-changing moves like Rain Dance, it could manipulate its own types.
Clearly, Game Freak intentionally tied the two together under the gimmick of weather changes. However, during a sandstorm, while Weather Ball would become a Rock-type move per its gimmick, Castform would not transform alongside this. Additionally, though it can't learn the move naturally, Castform can be taught the move Sandstorm via TM, further indicating that it always had the potential for a Sandy form. It could have even easily reflected the typing Weather Ball picks up and made its Sandy form a pure Rock-type Pokemon, or even a Ground-type if Game Freak wanted variety.
Possible Reasons Why There Wasn't A Sandy Form
No official reason has been given behind why Castform never received a Sandy form, but speculation can be made based on the idea behind the Pokemon and subsequent games. For example, when Castform was introduced it was allegedly an artificial Pokemon, making it one of an exclusive few like fan-favorite Porygon, and was therefore likely based on common weather types seen local to the Weather Institute in Hoenn where Castform originates from. Since sandstorms were localized to other areas like Route 111, the creators of Castform might not have accounted for this, potentially adding a curious piece of lore to the Pokemon.
Another reason why Sandy Castform might not exist could actually be more of a technical issue than an in-game explanation. While Game Freak could have easily added a Sandy form to round out the various weather effects, following games such as Pokemon XD: Gale of Darkness or Pokemon Diamond and Pearl introduced additional weather like shadowy aura and fog. Should Game Freak have set the precedent that Castform would receive a form for each weather effect, then any change to the weather system would create extra work for Pokemon designers.
However, Pokemon's weather remains loosely consistent between the overworld and battles to this day, where the weather in Sword and Shield such as fog or thunderstorm provide terrain effects. It's therefore unlikely that the introduction of a Sandy form would set a precedent that fans would expect future forms, especially given how malleable weather has become. In short, Sandy Castform could tidy up a loose end that's been left hanging in Pokemon's canon since 2002, and at a time when attention is always drawn to regional variants, a new form not region-exclusive could be a nice palate cleanser for once.