Dreg

In a possible timeline, Dregs were the mutated remains of those surviving humans who were left behind following a catastrophic war on Earth, now designated Orphan 55. (TV: Orphan 55)

Ryan Sinclair's worst nightmare was that he would travel with the Doctor for too long, and by the time he returned, the Earth would be destroyed and everybody living on it would have become Dregs. (TV: Can You Hear Me?)

Biology
Unlike their human ancestors, Dregs required carbon dioxide to breathe and exhaled oxygen as waste. Because of this, they were unable to breathe in oxygen rich environments for prolonged periods. Having evolved into the apex predators of the new ecosystem, they had fearsome fanged jaws and slept standing up.

The Dregs were also highly adaptive, able to survive the nuclear winter of Orphan 55, the low O₂, high CO₂ atmosphere of the planet and various forms of weaponry used by the Tranquility Spa staff to repel them, which required them to use randomisers on their laser blasters in order for their weapons to remain effective. Despite their adaptive nature, the Dregs were unable to breach the ionic membrane protecting the spa from them.

While they seemed to have abandoned many of the conventions of 21st century human civilisation (notably not wearing any clothing), the Dregs were demonstrably sapient creatures. They retained the memory of what had happened to their ancestors. Though carnivorous, they were also known to maim abducted victims simply for their own amusement.

The Dregs had some level of psychic ability, with a "pack" of them answering to the whims of a more powerful, more intelligent Dreg leader. (TV: Orphan 55)

History
The Dregs remembered their origin as the survivors of a nuclear war which had erupted on Earth, the result of tensions from the large-scale migrations forced by global warming. The Earth was classified an orphan planet and renamed Orphan 55 by interplanetary authorities, as it was believed to now be a dead, uninhabited world; the Dregs, the mutated, ruthless remains of humanity, had survived in secret.

After Tranquility Spa was illegally built in the Siberia of Orphan 55 by an entrepreneur unaware that it was still inhabited, the Dregs repeatedly attacked the resort, forcing it to protect itself with an ionic membrane. More than an intrusion into their hunting ground, the Spa provided the Dregs with a ready source of food and amusement in the sentients who came to rest there, whom they would abduct, torture and devour.

After Graham O'Brien won them a free holiday at Tranquility Spa, the Thirteenth Doctor and her companions uncovered the staff's cover-up of the Dregs' existence and crimes. Chasing after the Dregs in an ultimately futile effort to rescue abducted guest Benni, the Doctor came across a pack of Dregs and eventually entered their nest, where she was able to telepathically read the mind of a dormant Dreg and realise both its fierce intelligence and its true nature.

The immediate threat the Dregs posed to the guests of Tranquility Spa was eliminated by the destruction of the resort, and the Doctor and friends left the timezone in the Doctor's TARDIS. To her wondering companions, the Doctor explained that the Dregs were only one possible future of humanity, all of it depending on the choices made in the companions' own time regarding global warming; however, she could not guarantee that they would not come to be. (TV: Orphan 55)

In a nightmare induced by Zellin, Ryan Sinclair saw a Dreg as he found himself in a burning, future Earth. (TV: Can You Hear Me?)

Behind the scenes

 * The Dregs were designed by Javier Aliaga.
 * Only the Lead Dreg was actually made for use in practical effects, with the rest of the creatures added through visual effects, specifically via CGI.