Giant rat

Giant rats were created by Magnus Greel from common London rats using 51st century science, so that he could use them to guard the entrance to his laboratory located in the London sewers.

History
The Doctor shot and killed one as it attacked Leela. Li H'sen Chang, Greel's henchman, threw himself to the rats as a means of suicide after his master had dismissed him from service. The rats fatally wounded him though he did not die right away. (DW: The Talons of Weng-Chiang) After the defeat of Greel, Doctor's allies Henry Gordon Jago and Litefoot descended to the sewers with a Gatling gun to kill the rest of the rats still living there. (DWM: A Victorian Interlude)

The Doctor's eleventh incarnation mentioned giant rats to Amy as an example of things you could find on the London underground.(VG: City of the Daleks)