It is believed that man's inaugural domestication of any animal was that of the canine (Canis lupus familiaris) some 15,000 years ago. Since then, the animals have been bred and adapted into an astonishing variety to suit human purposes from hunting to Iditarod racing. Yet some have never forgotten their lust for blood.

Today, the vanguard of dog's manmade evolution can be witnessed in the urban habitat, where small breeds—most notably the pug and the Chihuahua—have been cross-bred into increasingly common new pedigrees: A pug crossed with a Chihuahua is commonly known as a "pughuahua," or "chug," a pug crossed with a beagle a "puggle," and so on into more elaborate genetic cocktails such as the "mini golden labradoodle."

Unlike their larger, more traditional brethren, the small urban canine mix is prone to behavior that deviates from that of a typical domestic dog. They may, for instance, become highly agitated and shriek when placed in a moving vehicle; they may tolerate—even enjoy—being dressed in sweaters and message T-shirts, as well as more extreme attire, such as lobster or SpongeBob SquarePants costumes; and may approach domestic cats more often with a shy curiosity and desire to be accepted than with the usual reciprocal hostility.

All of this careful breeding designed to fit comfortably into the world of the upwardly mobile urbanite belies the fact that even the pughuahua is descended from the gray wolf. Despite the years of monitored breeding, such seemingly remote ancestors of that noble beast can revert into dangerous feral predators that will happily turn on their owners—and each other—in a flash.

While the small urban canine mix is most commonly found in modes of docility—at doggy daycares, leashed to a chair on a patio during happy hour, being whisked around town in a bicycle trailer—it is at the off-leash areas of urban parks where one can catch a glimpse of the raw and deadly carnality of these blood...