Giant rats are a slight step up from sheep in terms of difficulty to kill. Naturally hostile these creatures are far larger than normal rats and tend to dwell on the docks and in dank, disease-ridden sewers.
Although weak, it is the only option (other than a sheep) for a player starting with taming level one. A rat can be used to kill other rats and sheep, and doing so (although it may take leveling more than one rat to level taming) will improve your taming skill unless it is capped at one.
Giant rats attack on sight and spawn frequently at the Lerilin docks, so be careful if you are fishing there!