Gentlemen,
Firstly, the pictures above-posted are indeed of a Caucasian Mountain dog, or more propoerly termed, a Caucasian Ovcharka.
Pictures were taken during temperment testing to see what this individual's drive and ability to engage was. (As this is a working breed, guarding instinct must be natural and strong) I was the decoy, and the individual dog was Caucasian Legend's Balou (rip).
A couple more pics:
They are indeed traditionally LGD's (Livestock Guardians) that have in recent history ben used as sentry guards.
THEY are NOT the "toughest breed" of dog... Whatever the hell that means.. They are very effective deterents and proven manstoppers, but are also a HUGE liability waiting to happen in the wrong hands...
Now, as far as Cougars...
In Argentina, there is a specialist dog breed, the Dogo argentino, whom was bred as a catch dog for both Boar and Puma.
Both in testing in a pen for young dogs before being brought into the woods ("Prueba de Campo"... "Proof of courage") as well as in actual hunting, a single Dogo has mastered and killed a single Puma many times.
One must keep in mind that these encounters usually result in extreme injury to the dog even if he comes away victorious.
In testing, the dog would not kill the cat, but master it to the ground. When the cat shows signs of submission, he would be split from the dog and the test concluded.
In hunting, the dog sometimes does finish the Puma, sometimes the hunter does.
One should keep in mind however, that Argentina's Pumas and North America's Mountain Lions are not nearly of the same size, the North American counterpart being significantly larger.
There have been reports of APBT's and AB's (American Pitbull Terrier and American Bulldog) killing Mountain Lion historically in the US. (Pete Sparks, publisher of the now defunct American Gamedog Times had a 40-50 pound APBT whom had killed a cat on record.) but the odds are significantly AGAINST the dog, going uphill against a bigger opponent.