Space pig

The "Space Pig" was a pig from Earth that was kidnapped and genetically modified by the Slitheen family into a humanoid form with an enhanced brain.

It was set into a spaceship launched from Earth in the North Sea and rigged to land in the Thames twelve hours later. This was intended as an excuse to put the British government on red alert for an alien invasion. It was shot and killed by a soldier while running around a hospital scared out of its wits, despite the Ninth Doctor wanting it to survive. (TV: Aliens of London)

Recent Torchwood recruit Owen Harper, a medical doctor, would have examined the space pig if not for his hangover. The medically inexperienced Toshiko Sato filled in for him and posed as a doctor instead. (TV: Exit Wounds)

Behind the scenes
The Space Pig was never named in Aliens of London, only referred to as "Alien" in the credits. The name Space Pig was given to it by Torchwood in Exit Wounds.