I nearly creamed two beagles coming around a blind corner on a mup one day. The person "walking" them was on a bike and didn't even have the leashes drawn in, despite the fact that it was a busy weekend afternoon. Luckily, I always slow right down and ring my bell for this particular corner as it's a sharp one.
That said, most dog owners hear seem to have more sense. When I ride to the pool before work, part of the route goes through a dog park. Yesterday my headlight picked up matching reflective vests on a german shepherd and it's owner long before I got to them.