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
One rat pursued Leela and managed to grab hold of his leg which is then bit. The Fourth Doctor shot and killed and killed it.

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)

Upon seeing the rats that Richard Harries was experimenting on, the Eighth Doctor recalled the attack on Leela. (EDA: The Banquo Legacy)

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