Killer dogs are not necessarily the only dangerous dogs. In some circumstances, any dog can be dangerous because all are “able or likely to inflict injury or harm.” Merriam Webster’s Collegiate Dictionary 315 (11th ed. 2005).

“Any dog can be dangerous.” (Heather K. Pratt, Comment, Canine Profiling: Does Breed-Specific Legislation Take a Bite Out of Canine Crime?, 108 Penn. St. L. Rev. 855, 858 (2004).

"All breeds of dog can and do inflict severe injury and death . . . .” (Safia Gray Hussain, Note, Attacking the Dog-Bite Epidemic: Why Breed-Specific Legislation Won’t Solve the Dangerous-Dog Dilemma, 74 Fordham L. Rev. 2847, 2848 (2006).

"It is common knowledge that horses buck, cattle roam, cats stray and dogs bite.” (Blaha v. Stuard, 640 N.W.2d 85, 88 (S.D. 2002).)

"It is the original nature of the horse to run and for a dog to bite.” (Sylvester v. Maag, 26 A. 392 (Pa. 1893).)

“Dogs generally cause harm by using their elongated snouts and numerous sharp teeth to bite. . . . A dog also can engage crushing power with his jaws . . . .” (Cynthia A. Mcneely & Sarah A. Lindquist, Dangerous Dog Laws: Failing to Give Man’s Best Friend a Fair Shake at Justice, 3 J. Animal L. 99, 136 (2007).)

Overly playful

A dangerous propensity can include dangerous playfulness and over-demonstrative affection:

"[O]ne who keeps a large dog that he knows to be accustomed to fawn violently upon children and adults is liable under [section 509 of the Restatement] for harm done by its dangerous playfulness or over-demonstrative affection .... [Par.] ... [L]ikewise [i]f the possessor knows that his dog has the playful habit of jumping up on visitors, he will be liable without negligence when the dog jumps on a visitor, knocks him down and breaks his hip .... " (Restatement Second of Torts, sec. 509, comments (c) and (i), cited in Drake v. Dean (1993) 15 Cal.App.4th 915.) 

"The owner or keeper of a domestic animal is bound to take notice of the general propensities of the class to which it belongs, and also of any particular propensities peculiar to the animal itself of which he has knowledge or is put on notice; and insofar as such propensities are of a nature likely to cause injury he must exercise reasonable care to guard against them and to prevent injuries which are reasonably to be anticipated from them. In this respect, a vicious or dangerous disposition or propensity may consist of mere mischievousness or playfulness of the animal, which, because of its size or nature, might lead to injury, for it is the act of the animal, rather than its state of mind, which charges the owner or keeper with liability." (Syl. pt. 6, Farrior v. Payton, 562 P.2d 779 (Haw. 1977).)

"[T]he law makes no distinction between an animal dangerous from viciousness and one merely mischievous or dangerous from playfulness." (Alex v. Armstrong, 385 S.W.2d 110, 114–15 (Tenn. 1964), quoting Owen v. Hampson, 62 So. 2d 245, 248 (Ala. 1952)).

Excessive number of dogs

Too many dogs might be dangerous; the City of Beverly Hills apparently thinks so, judging by this section of the Municipal Code:

Sec. 5-2.204. Walking dogs. It shall be unlawful for any person owning, controlling, or having in their care or custody, whether upon a leash or not, upon any public street, alley, or public place or upon any unenclosed land or property, four (4) or more dogs at any one time.  (5-1.206 Amd.) 

Training of attack dogs

Some jurisdictions have laws making it an offense to train attack dogs on residential property. The Virginia State Code contains the following provision:

3.1-796.93:2. Authority to prohibit training of attack dogs.

Any county with the urban county executive form of government may enact an ordinance which prohibits persons from training dogs on residential property to attack. As used in this section, "attack" means to attack or respond aggressively, either with or without command. Any such ordinance shall exempt from its provisions the training of dogs owned by any person who resides on the property. (1999, c. 848.)

Based on the authority given in section 3.1-796.93:2, the following provision was enacted in Fairfax County, VA:

Section 41.1-2-18. Prohibition on training of attack dogs.

It is unlawful for any person to train dogs, on property used for residential purposes, to attack or respond aggressively, either with or without command. However, this prohibition shall not prohibit a person from training dogs owned by such person on the property where the owner of the dogs being trained resides. (26-04-41.1.)