Mono Island is the home of the God of Evolution. As such, many of the other inhabitants are worried that if they hang around they'll become, somehow, improved, and go to great lengths to get themselves off the island. For instance, when Charles Darwin visited Mono Island (The Science of Discworld III) he had cut his hand so a bandage plant appeared nearby, hoping to prove useful enough to be transplanted.
Many of the denizens of Mono Island are beetles, due to the God's inordinate fondness for these creatures, but cigarette trees, wheeled elephants and dirigible whales may also be found.
The main landmark on Mono Island is Mount Impossible, which is hollow in order to accommodate the God of Evolution's creations and experiments, on which Ponder Stibbons worked for a short time (The Last Continent).
When the British scientist J.B.S. Haldane was asked what could be learned about "God" from the study of "His" creations, he purportedly replied, "...an inordinate fondness for beetles"