If you have seen BigDog and other robots from Boston Dynamics, then you need to complete your collection with WildCat, a fast running quadruped robot.
WildCat is another gasoline-engine-powered hydraulic quadruped robot similar to BigDog but based on the prototyping work of the Boston Dynamics' Cheetah robot. The Cheetah could manage 28 mph, but on a treadmill and it wasn't self contained. At 16 mph the new WildCat isn't as fast, but it is completely autonomous. It might be a mostly theoretical matter to comment that the Cheetah could out-run the fastest humans on the planet, whereas WildCat can't. In reality WildCat is going to be able to overtake most of us and, as long as its gas held out, it would even overtake the few athletes capable of running faster than 16mph once they approached their distance limits.
You can see it in action in the following video. It does look a little strange at first as its engine starts and it is covered by blue exhaust fumes.
"WildCat is a four-legged robot being developed to run fast on all types of terrain. So far WildCat has run at about 16 mph on flat terrain using bounding and galloping gaits. The video shows WildCat's best performance so far. WildCat is being developed by Boston Dynamics with funding from DARPA's M3 program. For more information about WIldCat visit our website at www.BostonDynamics.com."