Rat

A rat was a small rodent found on Earth.

In 14th Century, rats carried the Bubonic plague through-out Europe by their fleas. The Terileptil prisoner that found himself on Earth in the 1660s later made use of that by using rats to carry an enhanced version of the plague. (DW: The Visitation)

Magnus Greel experimented on one to make it as large as a horse to act as a guard to his underground laboratory. (DW: The Talons of Weng-Chiang)

One was used to show the effects of the "Sex Gas" to the Torchwood 3 team. (TW: Day One)