Ravenous

The Ravenous were a race of creatures from the Old Times who fed on Time Lords. A clan of Ravenous returned in the era before the Last Great Time War and were encountered numerous times by the Eighth Doctor, Liv Chenka, Helen Sinclair, and the Eleven.

Biology
In their natural state, the Ravenous resembled clowns, with the Doctor speculating that they inspired the traditional image of the modern clown as part of a universal racial memory attempting to make them less scary. (AUDIO: Deeptime Frontier)

The Ravenous were essentially the natural predator of the Time Lords, having evolved specifically to feed on regeneration energy; however, Time Lords suffering from regenerative dissonance, such as the Eleven, were essentially immune to the Ravenous, as their condition somehow contaminated their regenerative energy so that the Ravenous couldn't feed on it. (AUDIO: The Odds Against) They were also unable to feed on the Master when his essence was possessing a human body, as this process in some way contaminated his remaining regeneration energy to make it unpalatable (AUDIO: Day of the Master).

History
The Ravenous were originally a race known as the Kolstani, who existed on the planet Kolstan, a unique world that existed in the Time Vortex, the Kolstani drawing on the energy of the Vortex to sustain themselves, to the extent that they could warp reality to a minor degree. The Gallifreyan scientist Artron attempted to analyse that power to create a system of perfect regeneration, but was interrupted when the Doctor and the Master came to the planet, the Doctor seeking his insight into the Ravenous and the dying Master wanting his aid in restoring the Master's ability to regenerate. Having acted as Artron's assistant for several months, the Master devised a machine that would allow him to absorb the energy from the Kolstani, forcing Artron to help him by fatally wounding the Doctor, but Artron reconfigured the device to give him that power rather than the Master. However, draining the energy from the Kolstani compromised their mental stability and reduced them to a state of perpetual hunger, where they sought to take the regenerative energy from Time Lords as a means of compensating for their own loss of energy. (AUDIO: Day of the Master)

Old Times
In the Old Times of Gallifreyan history, according to the Matrix, the Ravenous threatened the earliest Time Lord pioneers. The Eighth Doctor speculated that the Ravenous evolved as the universe's way of balancing out the increased lifespans that Rassilon gave to his people.

One incident involving the Ravenous occurred during Time Lord interventions with the early human colony planet Yalta Prime. Rassilon ordered Time-Skiff Alpha 9 to materialise in Yalta Prime's atmosphere to awe the humans, but the Ravenous attacked the skiff during its journey and it arrived one week late without a single living crew member.

Rassilon led the near-genocide of the Ravenous. To retain moral high ground, the Time Lords kept a single family of Ravenous alive and imprisoned them in a bubble dimension which was lost in the Time Vortex. (AUDIO: Deeptime Frontier)

Imprisonment
Isolated in their prison dimension, the Ravenous learned "new tricks", including possession. (AUDIO: Deeptime Frontier)

Seeking that Time Lords be brought to them in their prison dimension, the Ravenous placed a bounty at the Maldovarium. Acting on this, the Nightstalker came to 1963 Shoreditch and attempted to kidnap the First Doctor and Susan Foreman for the Ravenous. He was stopped by River Song and Susan. (AUDIO: An Unearthly Woman)

Return of the Ravenous
Fearing the coming Last Great Time War, Daria Visteron used the Deeptime Frontier to release the Ravenous from their prison dimension. (AUDIO: Deeptime Frontier)

After defeating the Kandyman's and the Eleven's scheme with the thought spider, the Eighth Doctor and his companions caught of a glimpse of faces reflected in glass, screaming, and Helen Sinclair saw a word in her mind: "Ravenous." Afterwords, the Eleven mused to himself that he knew what they fed on, and that all anybody could do was run. (AUDIO: Sweet Salvation)

The Eleven was later found by one of the Ravenous onboard a dying TARDIS which he had stolen. The Eleven was scared enough to call for the Doctor’s help escaping it. According to the old tales, the Ravenous ate temporal energy, with Time Lords thus being their most potent source of "food", while shorter-lived life forms such as humans would be little more than the equivalent of a packet of crisps to satisfy the Ravenous' appetite. After the Eleven stole the Doctor's TARDIS, the Doctor was able to escape the Ravenous by using the last of the dying TARDIS's power to make a random materialization, leaving the Ravenous in the TARDIS as the ship's interior dimensions finally collapsed. (AUDIO: Seizure) This Ravenous' corpse was found by the Deeptime Frontier. (AUDIO: Deeptime Frontier)

The Ravenous found the Nine at the Nine's prison while pursuing the Doctor, (AUDIO: The Odds Against) and the two allied to trap the Doctor by creating a fake origin for the Ravenous. (AUDIO: The Odds Against, L.E.G.E.N.D.) The Eleven took the Nine's place in this partnership and tricked the Doctor into thinking the Ravenous had been destroyed. (AUDIO: The Odds Against)

With a Ravenous hiding in his mind, the Eleven eventually tricked the Doctor into taking him to the desert planet Parrak, which was said to be the location of Artron's tomb (AUDIO: Planet of Dust). Tracking down a copy of Artron's Matrix planet, the Eleven then returned to the Crucible of Souls, using Artron's knowledge to essentially reverse the polarity of the Crucible; where it had been originally designed to absorb the life energy from the dying universe, it would now grant regeneration energy to the entire universe, making them all capable of regeneration and providing more potential food sources for the Ravenous. Realising the threat this plan posed, three incarnations of the Master joined forces to stop the Eleven and the Ravenous by bringing the now aged-Artron to the Crucible, with the Ravenous hesitating to consume him due to their fragmentary memories of his past kindness. While the Masters reprogrammed the Crucible to remove the ability to regenerate from the rest of the universe, the Doctor was able to help Artron realise that he had to sacrifice his own power to restore the Kolstani to normal. With their normal power and mental stability restored, the Kolstani promised to take Artron and bury him somewhere peaceful in memory of his sacrifice. (AUDIO: Day of the Master)

Behind the scenes

 * Several Ravenous (including one in close-up) were featured in What is inside the TARDIS?, an animated, narrative trailer for Ravenous 4 released by Big Finish in 2019. Having invaded the Doctor's TARDIS, they were seen lurking in the rafters of the Eighth Doctor's console room's four metallic pillars before one of their number leapt down with a shriek.