Liability Apart From Trespass. For purpose of liability for harm other than trespass, the law distinguishes between animals domestic and wild. In the case of animals that are customarily domesticated and kept in that region (e.g., in the U.S., dogs, cats, cattle, sheep, horses, etc.) the keeper is strictly liable for the harm they cause only if he had actual knowledge (or had knowledge of facts which ought to have given him notice) that the animal had the particular trait or propensity which caused the harm. The trait must be a potentially harmful one, such as viciousness or destructive tendencies (as opposed to, e.g., excessive playfulness), and the harm must correspond to the knowledge； notice that a dog will attack other dogs is not, of itself, notice that he will attack humans. Thus, it is often said that “every dog is entitle to one bite,” but this is not necessarily true since the keeper may be on notice by reason of other known facts. In some jurisdictions, statutes impose absolute liability for certain types of damage (e.g. dog bites) without requiring scienter.
Keepers of species which are normally considered “wild” in that region (e.g., in the U.S., bears, lions, elephants, monkeys, etc.) are strictly liable for the harm they cause if they escape, whether or not the animals in question is known to be dangerous. And because such animals are known to revert to their natural tendencies, they are considered to be wild no matter how well trained or domesticated. However, where the injury occurs on the owner’s premises while the animal is confined or restrained, the cases tend to deny strict liability, commonly on a theory of assumed risk.