Rat

A rat was a small rodent found on Earth.

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 leath version of the plague, so that they could have the planet to themsleves (DW: The Visitation).

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).

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

Other information
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).