|The following is based on S.T.A.L.K.E.R.: Shadow of Chernobyl cut content and has not been confirmed by canon sources.|
Rats are quadrupedal animals of the rodent family. They appear in both S.T.A.L.K.E.R.: Shadow of Chernobyl and S.T.A.L.K.E.R.: Clear Sky. Like the Crow, it is a non-hostile animal which does not serve any significant purpose. Sakharov also has a rat in his laboratory in Shadow of Chernobyl that runs in a hamster wheel continuously.
Initially, however, the idea was to create Infected Rats which would look like monsters, and once located among the common Rats, bite them and rapidly turn them into similar monsters. This mechanic was cut from the game after it was clear that Infected Rats had much more aggro radius toward the player than the common Rats, hence this technique wouldn't have worked.