Additional forms change the appearance of a Pokémon, and may change their attacks. Unown even have their own official Pokédex addition to document all their different forms. Additionally, although not included in this list, two Spinda never look alike, their various spots making them as unique as snowflakes, each Smeargle has subtle differences in the color of their painting fluid, some regions have Pokémon with obvious pattern differences, and special "Vending Machine" Pokémon have been bred to have different attacks and appearances.
Retyped forms change the appearance and typing of a Pokémon, and may change their attacks. Arceus' golden array and hooves and Genesect's cannon have also both been known to change colors to indicate their multiple types, but are excluded from this list (and have their own list below instead).
Pokemon who only change colors, and not appearances, but do change types and/or have a signature move that changes types relating to their colors.
Alternative shiny colorations
Not to confuse with regional differences or coloration changes, these are rare shiny Pokemon who do not have the color of either their normal or standard shiny counterparts. Alternate shiny Pokemon are exceedingly rare and indeed, are only being discovered. All that is known is that not every Pokemon seems to have the genes needed for this coloration, and even evolution seems to randomly trigger or suppress the alternate coloration.