This video is called Kangaroo Walking.
From Wildlife Extra:
A fifth leg helps kangaroos walk
Red kangaroos may be one of nature’s best hoppers, able to lope along at speeds of up to 12 miles an hour on their hind legs, while their two front legs seem to dangle obsolete.
But when they are grazing or walking, which is actually most of the time, not only do they need those front legs but also their tail, which a new study has dubbed their fifth limb.
“We found that when a kangaroo is walking, it uses its tail just like a leg,” said study author Associate Professor Maxwell Donelan of Simon Fraser University in Canada.
“They use it to support, propel and power their motion. In fact, they perform as much mechanical work with their tails as we do with one of our legs.”
When grazing on grass red kangaroos, which are the largest of the kangaroo species in Australia, move both hind feet forward “paired limb” style, while working their tails and front limbs together to support and move their bodies.
“They appear to be awkward and ungainly walkers when one watches them moseying around in their mobs looking for something to eat,” said co-author Associate Professor Rodger Kram.
“But it turns out it is not really that awkward, just weird. We went into this thinking the tail was primarily used like a strut, a balancing pole, or a one-legged milking stool.
“What we didn’t expect to find was how much power the tails of the kangaroos were producing.
“It was pretty darn surprising.”
However when the roos are in their faster, hopping gait the tail returns to being a dynamic, springy counterbalance.