Reaper

Reapers appeared in the Ninth Doctor episode Father's Day. They are not named in the episode, but they are referred to as Reapers on several official sites.

Reapers are multi-limbed, flying reptiles similar to pterosaurs. They have a sleek head, four long hooked arms and a mouth on their chest. They apparently live in the Time Vortex that the TARDIS travels through. They are summoned when there is a temporal paradox, similar to the way bacteria swarm around a wound. They then sterilize the wound by devouring everything, with older objects being the tastiest. They engulf creature with their mouth, leaving no trace. They are material creatures though (at least they are in this world), and can be blocked. They can’t pass through material objects, with older objects being more effective. Unfortunately, the damages from paradoxes are accumulated, so they can be summoned inside these barriers.

In Father's Day, the paradox that summoned the Reapers was the survival of Rose Tyler's father, Pete. Pete eventually scarified himself to restore the timeline. While the death was still different, this was apparently enough, as the Reapers disappeared and those who had been absorbed came back alive.

The Doctor mentioned that when the Time Lords were around, there were ways to stop the creation of paradoxes and that any paradoxes that did manifest could be fixed. This implies that the Reapers are a natural phenomenon, though unwanted and uncommon.

The Reapers are similar in both appearance and habitat to the Vortisaurs from Storm Warning, the Hunters from The Pit and the Chronovores from The Time Monster and No Future.