Dog collisions are by far the biggest cause of crashes on the local cycle club rides. A very small dog caused a horrific crash on a downhill a few years ago.
Last fall, we rode past a farmhouse, and the owner warned us that her dog had gotten loose and was in the field ahead. Up around the bend in the road, oh great, here it comes, about a 40 pound mixed breed dog. We do the usual, yelling "go home", squirting water bottles, etc. Nothing affects him.
The dog follows us, and after a while, I see he stays behind or alongside on the grass, never running in front or near the riders. On a steep hill climb, the dog passes and goes ahead to follow the riders farther up the road. Then it falls behind when we ride downhill, rejoining on the next small hill. When a car passes, the dog gets off the road. When we stopped, he approached to get petted, and we see his collar has his name "Grizzly" and a phone number in big letters. Must not be the first time...
So after 5 miles (!) we worry that he's going to follow us right across a busy divided highway crossing ahead. So we borrowed a rope from a homeowner, tied him to a telephone pole, and called his owner to come get him. He was happy to just sit there in the grass as we rode away.
That was kind of fun, but it's really unique.