Teranesia - Greg Egan

As a young boy, Prabir Suresh lives with his parents and sister on an otherwise uninhabited island in a remote part of the Indonesian peninsula. Prabir names it Teranesia, populating it with imaginary creatures even stranger than the evolutionarily puzzling butterflies that his parents are studying. Civil war strikes, orphaning Prabir and his sister.
Eighteen years later, rumours of bizarre new species of plants and animals being discovered in the peninsula that was their childhood home draw Prabir's sister back to the island ? Prabir cannot bear for her to have gone out alone and he follows, persuading a pharmaceutical researcher to take him along as a guide.