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; the victim's life would change and the universe would compensate. For important people, like Donna Noble, whole new parallel universes could be created.

In a parallel universe created by the Beetle, it existed in a state of flux, and could not be touched. Though it was invisible to humans, some sensitive individuals were able to see it, particularly as major timeline changes were coming to pass.

The Beetle could also be made visible using the technology of the Lodestone. Once it was removed from Donna and stripped of its hold on time, the Tenth Doctor was able to directly observe and interact with a Beetle's corpse.

History
The Time Beetle claimed other victims before Donna Noble. (WC: )

A Time Beetle attached itself to Donna Noble and created a parallel world around her in which she never met the Tenth Doctor. This resulted in his death during the Racnoss incident, without her to stop him, and the subsequent deaths of Martha Jones, Sarah Jane Smith and the Torchwood Three team (apart from Jack Harkness, who became trapped instead).

The planet Earth, and Britain in particular, descended into chaos over the next two years in the aftermath of various alien attacks and catastrophes which the Doctor was meant to have stopped. Even the stars began going out, (TV: Turn Left) as Davros unleashed his Reality Bomb, uncontested. (TV: Journey's End)

With Rose Tyler's help, Donna went back in time to the beginning of the new timeline and stopped herself from turning right, restoring events as they had been. At this point, the Time Beetle released itself from Donna's back, and died from the tremendous force. (TV: Turn Left)

Before the Time Beetle attached itself to Donna, Lucius Petrus Dextrus prophesied to the "daughter of London" that "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)