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.

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

Galarian forms
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 however they do not have any sprite nor model assets.

Mewtwo
Mewtwo has an extra Armored Form. Normal Mewtwo is more offense-oriented while Armored Mewtwo is more defense-oriented.

Unown
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.

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

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

Burmy and Wormadam
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
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.

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

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

Shaymin
Shaymin has two different forms.

Arceus
Arceus has 18 different forms depending on its type.

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

Kyurem
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.