I've seen this. Utah's West Desert is a very desolate place, with millions of acres of undeveloped land. There is some private land, but a lot more public land. The whole corner of the state is mostly unused, except for some hunting and some scattered farms and ranches. Some ranchers run their livestock out there for open range grazing. A few areas are popular for off road recreational vehicles (motorcycles, ATVs, etc.)

Years ago a few friends and I were riding through on motorcycles, on the dirt road that started out as part of the Pony Express route in the 1860s. As we passed an area with a large flock of sheep (hundreds of them), two Great Pyrenees approached us. They were not wagging tails and coming up just to see what we were about, or to make new friends. They looked serious about keeping us away from the sheep, and they actually chased us off. I had been doing 20-ish MPH, taking it easy, enjoying the scenery, and trying to stay safe through the occasional pockets of loose sand (wasn't on a proper dirt bike). I had to speed up quickly to evade the dogs, and they didn't stop chasing until they were sure I would not pose a threat.

I have no doubt that two or three of these dogs could protect a large flock from coyotes, human intruders, or whatever might come along. I've even heard that they will attack a sheepdog that bites at the sheep. The sheepdogs keep the flock in a tight group and move them around as needed, but they're trained not to nip or bite, in spite of their instinct. The Pyrenees guard and protect the sheep, from sheepdogs as well as outsiders.