There are thousands of species endemic to Australia (found nowhere else). This can be attributed to the geographical isolation of the continent -- it is the most isolated of the continents, and was only discovered by non-natives in 1606. Until the construction of the Suez Canal, Australia was the only continent not to be connected by land to other continents. Australia has been isolated from other continents since it split from Antarctica about 40 million years ago.
The most famous species endemic to Australia are the marsupial fauna. Marsupials are mammals with a different lineage and anatomical differences from placental mammals, including a lower body temperature, early birth, lack of placenta, and different sexual organs. They include herbivores, like the koala, wombat, rat kangaroo, wallaby, and the famous kangaroo, the omnivorous bandicoot, Sugar Glider, marsupial mole, and assorted wallabies, the carnivorous quoll, Kowari, numbat, mulgara, antechinus, and many others. Altogether, 224 marsupial species are endemic to Australia. Marsupials have entirely displaced placentals within Australia.
Other species endemic to Australia are the monotremes, unusual mammals in their own category. They have a metabolism even slower than marsupials, though they have all the standard mammalian traits like hair and the ability to milk their young. Monotremes include four echnidna species and the platypus, considered one of the most unusual members of the animal kingdom. The platypus is described as duck-billed, beaver-tailed, and otter-footed. It has a venomous spur on its hind leg, and lays eggs, one of the only mammals to do so. Monotremes were poorly understood for many years, considered "more primitive" mammals. They split evolutionarily from other mammals about 150 million years ago, all the way back in the Age of Dinosaurs.
There are numerous other species endemic to Australia, although the marsupials and monotremes are the most famous. In Australia, about 83% of mammals are endemic, 89% of reptiles, 90% of fish and insects, and 93% of amphibians. The famous emu, a flightless bird related to the ostrich, makes its home there, and it is found on the coat of arms of Australia. Endemic parrots species are numerous on the continent. The Saltwater Crocodile, the largest of all living crocodile species, is found in Northern Australia, but can also be found in Southeast Asia. The Freshwater Crocodile, a much smaller species, is endemic to Northern Australia.