During your travels in Alola, you may come across Pokémon found in earlier games that have changed drastically to adapt to their Alolan habitats. These Pokémon are classified as Alolan Forms.
Alolan Form Pokémon share the same Pokédex number as the original Pokémon, but may have different stat distributions or types. Because Alolan Form Pokémon have become this way to match their surroundings, they cannot suddenly change to their originally discovered form and vice versa.
Furthermore, a small selection of unchanged Pokémon can evolve into Alolan Forms rather than their original forms. For example, a regular will evolve into the Alolan . It doesn’t matter where these Pokémon originate from, as long as they evolve while within the confines of Alola.
Essentially, Alolan Forms provide old–and often outclassed–Pokémon with a new lease of life. The biggest winners from moving to Alola are (great typing combined with bulky stats), (access to Aurora Veil and Snow Warning) and (fantastic defense typing). The rest are either marginably better, no different or worse.