Protip: stay clear of wombat lairs. Seriously. To quote Wikipedia:
"The wombat's primary defence is its toughened rear hide with most of the posterior made of cartilage. This, combined with its lack of a meaningful tail, makes it difficult for any predator that follows the wombat into its tunnel to bite and injure its target. When attacked, wombats dive into a nearby tunnel, using their rump to block a pursuing attacker. Wombats may allow an intruder to force its head over their back and then use their powerful legs to crush the skull of the predator against the roof of the tunnel, or drive it off with two-legged 'donkey' kicks."
And as if death by wombat butt wasn't bad enough, they can also charge, claw and deal really nasty bites. So yeah, don't mess with Wombat-kun.