About the Rat Things
Rat Things, also known as semi-autonomous guard units, were built to be the guard force in Mr. Lee's Greater Hong Kong. Rat Things were invented by Mr. Ng, of Ng Security Industries, who was severely handicapped after a helicopter accident in Vietnam, and, like the rat things, is also a cybernetic organism. Rat Things are genetically engineered American Pit Bull Terrier clones, spliced with rodent DNA and surgically augmented with cybernetic components. Rat Things are about the size of a Rottweiler, but have a long, flexible, whip-like rodent tail, which itself is cybernetically augmented.
Like other technology in Snow Crash, Rat Things are powered by a nuclear isotope power system, which requires extensive cooling. The Rat Things are cooled by a system of radiator-like fins over which air passes when they are running. When they are stopped, they must remain in their hutches (effectively dog houses), where they are cooled by jets of water sprayed over their cooling fins. When running, the Rat Things can travel over 700 miles per hour, and even break the sound barrier. When they were initially in captivity, they were forbidden from using this ability except in emergencies, but after escaping they constantly run at full speed all the time (they are no longer capable of staying still, since they no longer have specially cooled 'dog houses' to stay in), with devastating consequences for them and for everyone and everything around them.
Rat Things still have the basic mindset of dogs. However, their brains and endocrine systems are modified so that they are incapable of feeling empathy or any emotions other than anger, hatred, and fear. When in captivity, they were kept in a virtual reality program emulating the life experience of a normal dog, kept on an endless loop. They can also communicate with other Rat Things by "barking" via wireless transmissions. Their minds are largely controlled by their implants, but they can sometimes act independently of their programming, which ultimately led to their escape from the facility. (While they could easily jump over the fences that kept them trapped, they were programmed to believe that they couldn't, so for a long time they never bothered trying. When one Rat Thing finally tried and made it over the fence, the others all simultaneously realized that they could do it too, and they all broke free.)
It should be noted that even after their escape, they were still just following their programming and instincts. Unfortunately, the combination of their security programming, their perpetual fear and rage, and their natural predatory nature led them to become vicious and destructive killers, brutally murdering everything in their path.