OOC speculation, best I can figure is that the rats may have been accidental. They get into all sorts of things, including, perhaps, medical waste from the experiments that were conducted on Jewell Island in the 20th century. Also, the initial experiments are likely to have used rats, as, well, lab rats. If the experiments on the rats involved radical nootropic drugs, some lab rats may have gotten free as a result of the process working too well and then ended up breeding with the population of wild rats in the sewers below Manhanock Asylum for the Criminally Insane.
As Danny O'Toole demonstrates, the
children of test subjects may exhibit even more dramatic alterations than the original subjects. If this holds true for rats, then they could have been developing powers for generation after generation. Furthermore, if O'Toole is any indication, this process may result in individuals that are, frankly, giant tools. Also, I'm not abandoning my OOC theory that there is a superpowered, hyper-intelligent rat king in the tunnels of Jewell Island, ruling over the lesser rats with an iron... [squabbling mass of rat tails all tangled together?] In any event,
rat king. Fear his furry rodent Majesty!
Even more OOC, the superpowered mousers are a theory of the imagined progression based on precisely the same logic as your guess. In the theory, the first attempt to get rid of the rats used cats subjected to various performance-enhancing drugs and other experimental methods. I've decided that the cats proved a failure, however, being unable to work together effectively and thus falling prey to the uncanny coordination of the Rat King's rodent army.
There is actually some reason to believe in bull terrors, though. That is, we know [super-double spoiler] that there has been a Mad Scientist working in Manhanock Asylum even after the official closing of the laboratories there. [/end ultra-spoiler]. That individual, who shall remain unnamed, has been experimenting on dogs. Well, s/he was, before moving on to human subjects, as Science intended.
It shall not be revealed whether the 'lizards' that the guards in the tunnels have mentioned in the story as far as I've gotten in this thread are former test subjects of Dr. Edward Vanderbert or if they are created by a new Mad Scientist.