Under Agriculture & Markets Law §123, the owner of a dog previously adjudicated (or provable as) "dangerous" is strictly liable for the victim's medical and veterinary costs — no negligence proof required. But for the damages that drive case value — pain and suffering, scarring, lost income — New York follows the common-law rule: the victim must show the dog had vicious propensities and the owner knew or should have known of them.
"Vicious propensity" is broader than a prior bite. Growling and snapping history, lunging at passersby, being kept as a guard dog, "Beware of Dog" signage, complaints from neighbors or delivery workers — all are evidence of the propensity and the owner's knowledge. This is investigation work: canvassing neighbors, mail carriers, and prior complaint records in the first weeks, before stories soften. Notably, New York rejected a general negligence theory for dog injuries — propensity is the path, which makes that evidence collection decisive rather than helpful.