Fossil remains of Koala-like animals have been found dating back 25 million years. Some of the relatives of modern koalas were much larger, including the Giant Koala, Phascolarctos stirtoni.
It should likely have been named the Robust Koala, instead of Giant, but this big boy was larger than modern koalas by about a third. Phascolarctos yorkensis, from the Miocene, was twice the size of the modern koalas we know today. Both our modern koalas and their larger relatives co-existed during the Pleistocene, sharing trees and enjoying the tasty vegetation surrounding them.