How best to defend against a dog bite?

Don't know what he meant by a dangerous dog. Did you have immunization record for the dog? Did his insurance cover the injury? What is he trying do? Suing you? Every jurisdiction has some kind of ordinance for dog leashes. But as the old saying says, every dog is entitled to a free bite. If you did not know the dog's propensity at time, you were not negligent. But check local ordinance to see whether there is a dog leash law and if you broke it by not leashing the dog. That may be negligence per se. But without knowing whether the guy is trying to sue you, do not worry.