This is true. Last year I encountered a flock of sheep that were right on the trail shortly after Spring Creek Pass, right beside Antenna Hill. There were sheep right on the trail, and the flock stretched significantly on both sides of the trail. Undeterred, I kept walking, with the knowledge that livestock typically just get the hell out of your way when you approach them.
I was right. The sheep did get the hell out of my way, after a fair bit of bleating and the clanking of several sheep bells around their neck. However, what I hadn't realized is that the sheep were being guarded by three large dogs who were now very pissed off by my presence and by the fact that I had disturbed their flock. I soon found myself encircled by three angry, barking dogs.
This is the only time in my hiking career that I wished that I was packing heat. If I had had a gun with me on that day, there would have been at least one dead dog laying on the Colorado trail. However, all I had was my hiking poles, which would be fine if there was just a single dog, but they were quite inappropriate for taking on three dogs at the same time. I slowly made my way up the trail while being followed by my triad of snarling and barking escorts. Eventually, I got far enough away from the flock that they abandoned me and went back to guard the sheep.
If there was an unleashed dog with me, I am convinced that there would have been a very nasty dog-fight.
Here's a photo of the type of dogs that I encountered: