List of Pokémon with different forms

Some Pokémon may appear in different forms during certain weather conditions or in certain regions. These Pokémon have some differences comparing them to regular ones of the species, such as different types, moves, base stats and height and weight.

Regional forms
There are certain Kanto-region Pokémon that have different forms as they inhabit the Alola region.

There are certain Pokémon that have different forms as they inhabit the Galar region. There are also Galarian forms of Slowbro, Mr. Mime, Corsola and Yamask in game data and they are mentioned in respective Pokédex entries, however they do not have any sprite nor model assets.

Specific Pokémon forms
In Pokémon GO, Mewtwo has an extra Armored Form. Normal Mewtwo is more offense-oriented while Armored Mewtwo is more defense-oriented.

In case of Unown, there are no different types, moves nor statistics among any of its forms. Each of these forms resembles other letter of Latin alphabet with addition of exclamation and question marks.

There are currently 20 different patterns for Spinda and additionally 20 another different patterns for its shiny variants in game data, which gives total of 40 different patterns for Spinda. 8 of these forms were released so far.

Castform can appear in different forms when spawns during certain weather conditions.

Different forms of Deoxys specialize in different extents of base stats.

There are three different forms of Burmy and three respective forms of its evoltuion, Wormadam. While Wormadam forms differs for its type, Burmy forms differ only with appearance of its cloak.

Cherrim also has two different forms, which only difference is its appearance and the Weather condition for it to spawn. Sunshine form Cherrim spawns during sunny weather whilst Overcast form Cherrim spawns during every else weather condition.

There are two different forms for each Shellos and Gastrodon. West Sea form Shellos is available on the West Hemisphere and East Sea form Shellos is available on the East Hemisphere.

Rotom has six different forms that differ with its secondary types.

Giratina has two forms namely Altered Forme and Origin Forme. Altered Forme is more defense-oriented while Origin Forme is more offense-oriented.

Shaymin has two different forms.

Arceus has 18 different forms depending on its type.

Besides its Galarian form, Darmanitan on its own has two different forms. Zen Mode one is not available currently.

Each member of Forces of Nature has two distinct forms - Incarnate Forme and Therian Forme.

The other Unova-region Legendary Pokémon to have different forms is Kyurem, which in spite of regular form has also Black and White forms.

Trivia

 * On May, 2018, Alolan forms of Pokémon, originally discovered in the Kanto region, were stated to begin to appear in-game.
 * Some Event Pokémon also count as different forms rather than costumed Pokémon in game, thus they count towards the special trades if one of Trainers don't have such event Pokémon.
 * Shadow Pokémon and Purified Pokémon are also treated as different forms in game.