From Bulbapedia, the community-driven Pokémon encyclopedia.
Form differences are when, within one species of Pokémon, there are major, visible differences between individuals. Sometimes, this will as well extend to the Pokémon's types and stats, though this is not always the case.
In the games
Unown was the first Pokémon revealed to have visual differences between individuals of the species. Each individual Unown, at least in Generation II, would have a form that corresponded to one of the twenty-six letters of the Latin alphabet. In Generation III, two more forms were added, for the punctuation marks ? and !. The Unown Dex of Generation II also listed various words that corresponded to the Unown's different forms, though the differences between Unown appearance do not affect its stats.
Like Unown, Spinda has many visual variations among the species, involving the placement of its spots. Like Unown's letter, the spot placement is generated based on the personality value of each of the individuals. The location of the spots has no effect on the Spinda's stats. Unlike Unown, though, these differences are very rarely repeated, as they are based on random integers that are different for each of the spots on its face and ears. In fact, there are more than four billion different forms of Spinda.
Diamond and Pearl
The first Pokémon to change its type, as well as its appearance, was Castform. However, unlike other Pokémon, Castform does not retain its changes outside of battle, and can actually change as the battle is going on. Castform's current form is dependent on the weather in the battle: If the battle is being conducted in clear conditions, a sandstorm, or fog it retains its normal sprite and Template:Type2. If the sun is shining brightly, though, Castform will assume a more sunny appearance and the Template:Type2. During rain, it will become reminiscent of a raindrop, and will assume the Template:Type2. Finally, while it is hailing, Castform will become more icy, and become an Template:Type2 Pokémon. The type will change at any time the weather does.
Deoxys is the first Pokémon to have its actual base stats, as well as appearance, change depending on the form it takes, with Defense, Normal, Attack, and Speed forms. They are game-specific, with the Defense form appearing only in LeafGreen, the Normal form appearing only in Ruby and Sapphire, the Attack form appearing only in FireRed, and the Speed form appearing only in Emerald. All four forms can be found in Diamond and Pearl, and changed at will by inspecting the fallen meteors in Veilstone City.
Burmy and Wormadam
Both Burmy and its female evolution, Wormadam, have three separate forms, though only Wormadam's form makes a difference on its type. Burmy's form changes after each battle it participates in: if it last battled outside, it will be covered in leaves, if it was in a building, it would assume a form that is seemingly covered in insulation, and if it last battled inside of a cave, the it would have what seems to be dirt and rocks on its exterior. Wormadam's three forms are similar, however, the form is permanent, and the type that she will be either Bug/Grass if she is in her leafy form, Bug/Steel if she is in her insulation-covered form, or Bug/Ground if she is in her rocky form. Her stats and move compatibility vary as well.
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 in the east or west of Sinnoh. The western variation of both is pink, while the eastern variation is blue. There is also some difference in their bodily appearance whether they are an East Shellos/Gastrodon or a West one.
In mid-February, 2008 it was announced that Giratina would have two different forms in Giratina and the Sky's Bouquet: Shaymin. One of these is the familiar form that appears in Pokémon Diamond and Pearl and in Ken Sugimori's artwork, known as "Another Form", while the other is a new, more serpentine form known as the "Origin Form", which was later revealed to be the form Giratina takes during Pokémon Platinum, the third version of Generation IV.
The seeming god of all Pokémon, Arceus has multiple forms with slight palette differences that represent the seventeen elemental types. These depend on the plate which Arceus holds. Strangely, there is a Template:Type2 Arceus sprite, but no possible plate to make it this type.
The Pokédex entry for Arbok states that the markings on its belly differ from region to region. Likewise, in Generation IV, there are many Pokémon with slight sprite differences based on their gender, and even one, Cherrim, which changes depending on the time of day. Lastly, Mantyke's Pokédex entry in Pearl says that the special patterns on its back vary from region to region. However, this has yet to be proven, since this Pokémon only has been catchable in one region during the latest generation.
Other form differences
There are several other differences that Pokémon have. For detailed descriptions of them, see Variant Pokémon.