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