Specifically, this is the mating call of a male koala.
A recent study found that what allows these fairly small animals to make such a remarkable noise is the length of their vocal anatomy and peculiar shape of their larynx. It's actually more similar to a humans than any other marsupial and is what allows the males to sound far larger than they really are. In fact the larynx morphology (called a "descended larynx") was once thought to be only found in humans and vital to our ability to speak. Now, however, it has been discovered in both koalas and also red deer. I would suspect that eventually other animals will be found to have the same unique feature. Sounding big is probably an important part of the mating rituals of these interesting marsupials, likely scaring away other males.