Rats (a.k.a. Pandyssian Bull Rats) are enemies encountered in Dishonored both in swarms or alone. They're the carries of the rat plague, spreading the disease, which infects the inhabitants of the city of Dunwall.
Rats can be encountered throughout Dunwall, both in swarms and on their own. They are aggressive in swarms, and large groups are more likely to attack Corvo if he nears them. While they are cowardly in smaller numbers, in swarms they can easily take down and consume a lone watchman in a matter of moments.
Rats congregate in dark places and wherever corpses can be found. Although they are deterred by bright lights and large gatherings of people, they will notice if the former fails, and will investigate any newly-available areas.
The noise and spread of a pistol is a good way to quickly disperse a crowd of rats, killing some of them in the process. However, it is ill-advised to stay nearby for too long, as the dead rats will respawn with any remaining rats and will swarm at Corvo once more. An easy but time-consuming way to get rid of rats is by standing on higher ground, such as a chair or table, and hacking at the swarm of rats gathering at Corvo's feet. They will be unable to damage Corvo while he can hack freely at them with his sword.
Also, a swarm can be annihilated by throwing a grenade in the middle of them. Any other thrown objects (Bricks, bottles etc.) will work too albeit not as effective as a grenade.
Utilizing the Rats
Corvo can utilize these creatures in two ways. He can possess a lone rat to sneak past the guards unnoticed or fit the rat holes which can get him to another area faster. The other way is that Corvo can summon a swarm of rats from the void which will attack and consume anything on their path. Summoned rats however will not attack the weepers (also true for normal swarms) and Granny Rags.