The marsupial lion (Thylacoleo carnifex = "meat-slicing pouch-lion") was exactly what it sounds like: a lion with a pouch.
Actually, to be more precise, the marsupial lion was not a lion at all, but a distant relative of kangaroos and wallabies that lived in the Pleistocene epoch (ca. 1,600,000–45,000 years ago) and ranged across all areas of Australia. But damned if it didn't look exactly like a slightly undersized lion, and filled the same ecological niche too, in a case of convergent evolution.
The marsupial lion was about 3 feet tall and 5 feet long, and weighed in at about 350 pounds. It also had several unique traits that made it a devastating apex predator, including retractable claws (the only marsupial known to have this), the ability to stand up on its hind legs and balance with its tail like a kangaroo while slashing at large prey with its arms, opposable thumbs (!!!), presumably for grasping its prey better, and a unique kind of blade-like molar in the sides of its mouth, both on top and bottom, that was more like a meat cleaver than a tooth, and acted like two pairs of garden shears for slicing up meat.
According to biometric calculations based on measurements of fossilized jaws, the marsupial lion had massive jaw muscles and pound-for-pound, the strongest bite of any mammal ever discovered.
Truly, the marsupial lion was a fearsome predator with no equal on the Australian continent. Until humans came. After dominating the continent for over a million years, the marsupial lion met its match when humans began arriving around 45-50,000 years ago. Although the humans were soft and squishy and did not have retractable claws or molars like garden shears, they did stand on their hind legs and have opposable thumbs. They also practiced a primitive form of slash-and-burn agriculture, which rapidly denuded Australia of the marsupial lion's plush forest habitat, and hunted much of the marsupial lion's primary prey, such as the massive Rhinoceros Wombat, to extinction.
By about 40,000 years ago, the marsupial lion disappears entirely from the fossil record.