Castform was added into Pokémon Go yesterday, alongside 20 more Pokémon from the Hoenn region.
With Castform's inclusion, Niantic has introduced a new mechanic into the game based off of its forms that Pokémon Go players have never seen before.
Castform has four different forms that players can catch in-game: Normal, Sunny, Rainy, and Snowy. When a trainer captures a Castform, it will permanently remain in that form.
Castforms in the wild, however, will change their form depending on the weather in the area. If the weather is cloudy, windy, or foggy, it will be in its Normal form. If the weather is clear, Castform will appear in its Sunny form. If it's raining, Castform will be in its Rainy form, and if there's snow, this Pokémon will change to its Snowy form.
The Pokémon Unown has a similar form feature in-game with over 28 different letters and punctuation marks to collect from the English alphabet. Castform's form feature, however, is the first to use weather as the determining factor behind its look.
Castform was originally teased in a datamine one week ago alongside Deoxys, who hasn't been added to the game yet. Deoxys also uses forms like Castform, but it's unclear how Deoxys forms will work in Pokémon Go.