Skippy the bush kangaroo
These are Western Grey Kangaroos (Macropus fulginosis). They are an extremely common and iconic animal in Australia. And yes, they are capable of killing you (via those foot claws to your belly resulting in disembowelment, but honestly, they're more likely to hop away if you scare them/piss them off). They are also tasty when served rare.
Anyway, none of that is WTF. But the 'Roo is capable of a few amazing things.
Kangaroos do a nifty thing known as embryonic diapause. Once a joey is born, the mother roo mates again immediately. By the time this second joey is born the first is hopping around out of the pouch a lot of the time and the new joey (at this stage just a "jellybean") is locked onto a teat in the pouch. Mama Roo mates once again, but this one she pauses at the embryonic stage. It sits in a kind of suspended animation. If conditions are hard and one of the two joeys she already has dies, she'll start the internal one developing again - it's a kind of insurance policy. If it's a good year and both first joeys survive, she reabsorbs the embryo.
Kangaroos also produce different "flavours" of milk, actually different constitutions of milk to be fed to joeys at different stage. One teat is secreting a suitable milk for the pinkie in the pouch, while another teat is giving milk that's good for a joey at foot. Nifty!