Thylacoleo

Thylacoleo Carnifex was a Marsupial, and the largest Australian (Mammal) predator.

Appearence
It's a commonly held misconception that that the giant wombats, kangaroos and koala bears of Pleistocene Australia were able to prosper thanks to the lack of any natural predators. However, a quick glance at Thylacoleo (also known as the Marsupial Lion) puts the lie to this myth; this nimble, fanged, heavily built carnivore was every bit as dangerous as a modern lion or leopard, and it possessed the most powerful bite of any animal its size--whether bird, dinosaur, crocodile or mammal--in the annals of natural history. (By the way, Thylacoleo occupied a different evolutionary branch from saber-toothed cats, exemplified by the North American Smilodon.)

Behaviour
As dangerous as it was, though, Thylacoleo may not have been the apex predator of its time and place--some paleontologists claim that honor belongs to Megalania, the giant monitor lizard, which may have hunted (or been hunted by) the Marsupial Lion. In any case, Thylacoleo exited the history books about 40,000 years ago when the earliest human settlers of Australia hunted its natural, gentle, herbivorous prey to extinction. (It's possible that early humans targeted Thylacoleo directly, but one doubts even a caveman would take such a foolhardy risk!)

In Prehistoric Park
Nigel Marven considers saving this animal, when looking for a Cat-like creature to save. He stated that it was an excellent climber, and ambush predator.