Of course if you really want to scare yourself, the current biota is what has been left as relatively harmless after 60,000 years of terraforming by the Aboriginal colonists. When they arrived the landscape was rather less friendly. Carnivorous Kangaroos 12 feet tall, huge flightless carnivorous ducks, (non-burrowing) wombats the size of a VW Beetle. Thylacoleo Carnifex, a kind of giant super possum evolved to kill and eat anything that moved. ... Monitor lizards the size of a stretch limo (the largest reptiles on earth at the time). Possibly even pack hunting land crocodiles (dating is a bit vague on those).
Which is just both utterly terrifying and awesome all at once. Sounds like an alien planet on a scifi novel!