The Time Beetle was an insectoid creature resembling a giant stag beetle that was capable of manipulating time. It was used by, or was part of, the Trickster's Brigade. With the help of humans within the brigade, or at least 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.
The time beetle 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 visible in the Circle of Mirrors.
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)