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 with a Chinese fowling gun.

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) Henry Gordon Jago and Professor George Litefoot later went down into the sewers with a Gatling gun to kill the rest of the rats. (PROSE: A Victorian Interlude)

Other references
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 Pond as an example of things that could be found on the London Underground. (GAME: City of the Daleks)

Behind the scenes

 * It is likely that the giant rat in The Talons of Weng-Chiang was inspired by a line in the Sherlock Holmes story The Adventure of the Sussex Vampire by Sir Arthur Conan Doyle, in which Holmes alludes to "the giant rat of Sumatra, a story for which the world is not yet prepared". In Talons, the Doctor dons a deerstalker and cape, and is assisted by a medical doctor whose housekeeper is named Mrs Hudson, all of which appear to allude to Conan Doyle's detective.
 * In his work of modern British history, State of Emergency, Dominic Sandbrook called the giant rat "one of the worst-realised monsters not merely in the show's history, but in the history of human entertainment.”
 * In an ironic allusion to Leela's encounter with the giant rat, Home Assistant shows actor Louise Jameson possessing a copy of Rat Monthly magazine.