The "Space Pig" was a pig from Earth that was kidnapped and genetically modified by the Raxacoricofallapatorian Slitheen family into a Humanoid form with an enhanced brain and set into a spaceship launched from Earth in the North Sea and rigged to land in the Thames 12 hours later, as an excuse to put the British government into red alert for an alien invasion. It was shot and killed by a member of the army (possibly a member of UNIT) while running around a military base scared out of its wits, despite the Doctor wanting it to survive. (DW: 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 then filled in for him. (TW: Exit Wounds) Doing so, Toshiko met the Doctor. (DW: Aliens of London)
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 by Torchwood in Exit Wounds.