The Chimera was a human spliced with alien DNA. It induced a primal fear in those who encountered it. Its basic form looked like a human with incredibly damaged, grey skin. However, it only appeared to operate in this guise when no one was watching. Once, it took on the shape of a black goblin with red eyes (to Jack Harkness), a zombie clown (to Gwen Cooper), a bearded man in a trench-coat (to Jack Harkness), a deceased piano teacher (to Owen Harper) and an approximation of a Cyberman (to Ianto Jones).
The DNA also gave it the ability to move incredibly quickly, increased strength and the ability to jump great distances without bodily harm. It could track someone using only the scent of their fear. (COMIC: The Legacy of Torchwood One!)
Circa early 2006, the Bio-Medical Research Division of Torchwood One discovered alien DNA that would induce a primal fear in those exposed to it. The Institute began kidnapping dead-beats and drug addicts off the streets, splicing their DNA with the alien's to create a super-soldier for the British Special Forces — "Bringing terror to the terrorists."
The experiment failed many times, each time causing the death of the test subject. One test subject was a success. However, it soon became uncontrollable and tried to kill its creator, Rupert Howarth.
The creature escaped but kept trying to kill Rupert, leading him to fake his own death to protect his family and friends. Before he left, he left a program running on his computer to find the formula to a solution to suppress the Chimera's alien DNA. The solution was stored on the Torchwood mainframe.
In mid-2008, the presence of the formula led Rupert to Cardiff, causing the Chimera to follow. After burning down the warehouse in which Rupert had been sleeping, it followed him to the Hub where Rupert, with the help of Torchwood Three's staff, created the solution.