Pack dogs are the most dangerous by far. When I lived in Iowa there was a case of some sort of horrible creature tearing assunder local sheep and other farm animals. It turned out it was a pack of DOMESTIC dogs. Dogs that returned home every morning to their loving families. Dogs that the owners claimed would never do such a dastardly thing, and thus it could not be their dog responsible. Nonetheless, it was determined to be several dogs from multiple farms and the owners had to pay hundreds of dollars in lost "property". Good thing you weren't riding in that area earily in the morning.
Here in The Dalles one cannot walk a mile without encountering half a dozen loose dogs. Getting growled at is a weekly occurance. I work at an elementary school and any kid who walks has encountered neighborhood dogs who have formed a small pack and chased them home. It is even more fun when these packs come on school grounds during recess. There is a report of a dog biting someone EVERY SINGLE FRICKING DAY in the paper. And there are only 12,000 people here. I'm actually afraid to go outside, let alone bike. Animal control claims there is nothing they can do if you don't know the address. How about come get the dog! Even when the owner does get reported the dog still runs free. If they do impound it, the lazy owner doesn't bother to get it and it is euthenized. It's cheaper and cuter to just get a new free puppy. Maybe one of these days I'll get lucky and catch a glimpse of the dog's tags as it rips out my jugular.
On the funny side, the only dog to ever bite me was a toy poodle. I'm telling you, anything biting you on the achilles tendon hurts like hell. And they're too fast to kick...