It's been a common fact that people in different environments have been transformed by animals, causing them to devolve to become exact copies of them, or just their own color when the same species. Basically 6 areas in the world have been considered to have transformative animals that could make someone's body change.
Those areas are, the mountains, the grasslands, the rainforests, the tundra's, the swamps, and the deserts. Anyone of these areas have different animals that would be capable of changing a human being, but the humans of said environment do not know what that animal could be when they see it. As those who travel to one of these environments, they were about to experience the changes 1st hand in front of them while they become the same size, and length as them all together.