Time Beetle

The Time Beetle was an insectoid creature resembling a giant that was capable of manipulating time. According to the Tenth Doctor, it was "one of the Trickster's Brigade". With the help of a fortune teller, it was attached to unsuspecting victims, after which it changed a key event in their lives. In most cases, this would have little impact. Their lives would change and the universe would compensate. For important people, like Donna Noble, whole new parallel universes could be created.

In the parallel universes created by the Beetle, it existed in a state of flux; it could not be touched. It was also invisible to humans, except to very sensitive ones. It was also made visible using the technology of the Lodestone. In the main universe, the Tenth Doctor interacted and observed a Beetle's corpse.

A Time Beetle attached itself to Donna Noble and created a parallel world around her in which she never met the Tenth Doctor. With Rose Tyler's help, she went back in time to the beginning of the new timeline and stopped herself from changing it. At this point the Time Beetle released itself and died from the tremendous force. (TV: Turn Left)

Before Donna was attacked by the Beetle, Lucius Petrus Dextrus propesied to the "daughter of London", "There is something on your back." (TV: The Fires of Pompeii)

Behind the scenes
The Time Beetle, with its position on its host's back, was inspired by the Eight Legs of Metebelis III. (CON: Here Come the Girls)