Hawaii is the most isolated group of islands in the world and has one of the highest endeminism rates in the world. Other animals are just considered subspecies of mainland animals. Some plants and animals grow larger than there mainland counterparts due to having different habitats/ food/ the absence of previous predators. Some even colonised and adapted due to Hawaiians changing the landscape.
I love that so far Alola actually reflects this with the Alolan versions and pokemon adapting to microclimates and pokemonl like
Oricorio who are adapted to different islands.