Rat

Rats were small rodents found on Earth. They were considered a pest by the Humans.

In the 14th century, rats carried the Bubonic plague throughout Europe by their fleas. Terileptil prisoners who found themselves near London in the 1666 exploited the fact, and infected the rats to carry with an enhanced, especially lethal version of the plague, so that they could have the planet to themselves. (DW: The Visitation)

Rats were used in experiments as test subjects, since they could be bred quickly. In the late 19th century, Magnus Greel made some rats into giants in order to guard his underground laboratory, which he accessed via the London sewers. (DW: The Talons of Weng-Chiang) A lab rat was used by Owen Harper to demonstrate to the Torchwood 3 team the ultimately lethal effects of the "Sex Gas" on its host. (TW: Day One)

In the early 2000s, undercover Krillitane at Deffry Vale High School freeze-dried rats in order to eat them later. (DW: School Reunion)