The Entity (TARDIS)

The Entity was a golden ball of energy who was imprisoned by the Doctor.

Biology
The Entity fed on the time energy of living beings, until the creature died of old age. The process took about an hour on the average Human. Whilst the Entity was feeding, the individual it was feeding on appeared to have a golden "shell". On its own, the Entity appeared as a golden ball of energy floating in mid-air.
 * It is unclear whether the Entity is a unique being or a part of a whole species of Entities.

History
At some point in his past, the Doctor met the Entity, and placed it inside a vase in a safe in the TARDIS drawing room.

When the Doctor was sucked into a time rift, Amy looked in the Doctor's drawing room for a tractor beam device to pull him back inside the TARDIS. Unfortunately, she knocked down a vase containing the Entity, and the Entity escaped.

Later, when the Doctor was back in the TARDIS, the rift caused Amy to be pulled into the TARDIS one thousand years in the future, where the Entity started feeding on her. The Doctor used a device to pull Amy back to the present TARDIS, and the Doctor told the Entity to stop feeding on her. The Entity agreed, and tried to feed on the Doctor. However, the Doctor trapped the Entity in another vase and told it that he would release into the time vortex with the Chronomites, creatures that the Entity could feed on for eternity. The Doctor released the Entity to the Chronomites, but forgot to mention that the Chronomites were very itchy. (VG: TARDIS)