From Bulbapedia, the community-driven Pokémon encyclopedia.
Form differences are major, visible differences between individuals within one species of Pokémon. Sometimes, this will as well extend to the Pokémon's types and stats, though this is not always the case.
Certain forms have specific names, and are usually specific to one or two Pokémon. When form differences concern legendary Pokémon, the forms are named as "Formes".
In the games
Unown was the first Pokémon that was revealed to have visual differences between individuals of the species. Each and every Unown, from the species' debut in Generation II, would have a form whose appearance corresponds to one of the twenty-six letters of the Latin alphabet. In Generation III, two more forms were added, which correspond to the question mark and exclamation point.
The Unown Dex of Generation II listed various words that corresponded to the different forms, though the stats of the Unown are not dependent on which form it takes. In Generation III, Unown's form is determined by its personality value.
Like Unown, Spinda has many visual variations in its species, all involving the placement of its spots. Like Unown's letter, the spot placement is generated based on the individual's personality value, and likewise, has no effect on the Pokémon's stats. Unlike Unown, however, these differences are rarely repeated, as the calculation uses the entirety of the personality value—a number that can range from zero to more than four billion.
The introduction of Castform in Generation III brought about the concept of interchangeable forms. Unlike other Pokémon with alternate forms, Castform changes its appearance, as well as its type, with the weather. If the battle is being conducted in clear conditions, a sandstorm, or fog it appears in its Normal Form and is a Template:Type2. If the sun is shining brightly, though, Castform will assume its Sunny Form and the Template:Type2. During rain, it will transform into its Rain Form and assume the Template:Type2. Finally, while it is hailing, Castform will change into its Hail Form and become an Template:Type2 Pokémon. The type and form will change at any time the weather does, and Castform will return to its Normal Form once the battle has concluded.
Deoxys, the final Pokémon of Generation III, is the first known Pokémon to change its base stats and move compatibility in addition to its appearance depending on the Forme it takes, with its Defense, Normal, Attack, and Speed Formes.
In Generation III, each of Deoxys's Formes is game-specific, with Normal Forme only being accessible in Ruby and Sapphire, Attack Forme appearing only in FireRed, Defense Forme only appearing in LeafGreen, and Speed Forme appearing only in Emerald. When the five games are linked, the different Formes can battle each other using their specific stats, however, if traded among games, Deoxys will become the Forme it appears in in that game.
All four of Deoxys's Formes appear in Generation IV, and can be freely changed by inspecting the meteorites in Veilstone City.
Burmy and Wormadam
Both Burmy and its female evolution, Wormadam, have three separate forms, being the first Pokémon whose evolution depends on a specific form. Like others, Burmy can change form freely, depending on where it last participated in a battle actively (Exp. Share maintains a Burmy's form if it does not leave the Poké Ball), however, Wormadam's form is permanent.
If a Burmy last battled or was hatched in a grassy outside area, it will be in its basic state, the Plant Cloak. This form of Burmy is covered in leaves and twigs, and if female, will eventually evolve into Wormadam in its Plant Cloak, a dual-type Bug/Grass Pokémon.
After battle on a beach or in a cave, or if it is hatched there, Burmy will be in a sand-and-rock-covered form, its Sandy Cloak. If a female, it will eventually become Wormadam in its Sandy Cloak, which is, rather than Bug and Grass, a Bug/Ground Pokémon.
Finally, if a battle has been conducted inside of a building, or if this is where Burmy hatches from its egg, it will be in a pink insulation-covered form, the Trash Cloak. Like the other forms, if it becomes Wormadam while in this form, it will be the Bug/Steel Wormadam Trash Cloak.
A Wormadam's stats and move compatibility vary with her form, in addition to type and appearance. The cloak a male Burmy takes on before evolution makes no difference in the long run, as it sheds it upon evolution into Mothim.
Like Castform, Cherrim changes its appearance depending on the weather, though not with all kinds. It will be in its normal Overcast Form if the weather is anything but sunny, and its Sunshine Form when the sun shines brightly. Due to its ability, its Attack and Special Attack will also increase in its Sunshine Form. Unlike Castform, however, its type does not change.
Shellos and Gastrodon
Another Sinnoh-native evolution family, Shellos and Gastrodon also have a difference, depending on, this time, whether the Pokémon in question was caught east or west of Mt. Coronet. Like others before, neither form can change to the other, but unlike other multiple-form Pokémon, the appearance of the mother is the determinant to the appearance of the hatched Pokémon.
Both West Sea Shellos and Gastrodon take on a pink appearance, while their East Sea cousins take on a blue hue. There is also a slight difference in bodily appearance depending on the form.
In August of 2008, shortly before the Japanese release of Pokémon Platinum, five silhouetted Pokémon were shown, with a statement that indicated that these Pokémon would be in the game. Days before the game's release, it was revealed that these Pokémon were alternate forms of Rotom.
In each of these forms, Rotom has possessed a different household appliance, and each has its own special move. Heat Rotom, which is based on an oven, can have Overheat, Wash Rotom, based on a washing machine, learns Hydro Pump on assuming that form, Frost Rotom, a refrigerator, learns Blizzard, Spin Rotom, based on a fan, learns Air Slash, and Cut Rotom, based on a lawnmower, learns Leaf Storm.
Despite this new variance in move types, all forms of Rotom retain their Ghost/Electric typing, though all alternate forms have a slightly higher base stat total than Normal Rotom.
Giratina, much like Deoxys had been earlier, was announced to have an alternate form in the then-upcoming Giratina and the Sky Warrior in February, 2008. One of the two forms is the familiar form to players of Pokémon Diamond and Pearl, designated as Altered Forme Giratina, while the other is a more serpentine Forme that was not previously known, the Origin Forme.
The Origin Forme is known as Giratina's natural Forme, which it appears in in its home of the Reverse World. If it travels to the Pokémon world, it assumes its Altered Forme. Both Formes appear in Pokémon Platinum, where Giratina is captured in the Distorted World, its home, in its more offense-oriented Origin Forme, and will revert to the defensive Altered Forme outside of this area unless it holds the Platinum Orb.
As with Giratina, an alternate Forme of the event Pokémon Shaymin was announced to appear in Giratina and the Sky Warrior, and is, in fact, the sky warrior as described in the title. This new Sky Forme of Shaymin, with the previously-known form now called the Land Forme, has a completely altered appearance and, rather than being a pure-Template:Type2, is now Grass/Flying.
Like Giratina and Deoxys's transformations are activated by an external source, Shaymin's is activated by contact with a Gracidea Flower, whose bouquets look almost like a sleeping Shaymin.
Arceus has eighteen different Formes, each with a slightly different coloration, which correspond to the eighteen different elemental types.
Arceus's transformation between these Formes is brought about by its held item: if it is holding a plate, it draws its type from that plate as stated in Sinnoh's mythology, while remaining Template:Type2 when not holding a plate or holding any non-plate item. The ???-type Forme of Arceus is currently unavailable in any manner, as there is no plate to turn it to this type.
Other form differences
There are several other differences that Pokémon have. For detailed descriptions of them, see Variant Pokémon.