Giant rat

Giant rats were created from common London rats by Magnus Greel in the 19th century, using 51st century science. He then used the rats to guard the entrance to his laboratory in the London sewers.

History
One rat pursued Leela and bit her leg. The Fourth Doctor shot 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. (TV: The Talons of Weng-Chiang) After Greel's defeat, the Doctor's allies Henry Gordon Jago and Litefoot descended into the sewers with a Gatling gun to kill the rest of the rats. (PROSE: A Victorian Interlude)

On seeing the rats that Richard Harries was experimenting on, the Eighth Doctor recalled the attack on Leela. (PROSE: 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. (GAME: City of the Daleks)