Haemovore time stream

In one timestream which was a possible future relative to 20th century Earth, humans evolved into a species of vampires called the Haemovores, having been mutated by the chemical slime of a polluted Earth. This time stream was accessible to time travellers. (PROSE: Goth Opera, TV: The Curse of Fenric) The Twelfth Doctor was unable to recall if the future engineered by Fenric was an "alternative timeline" or "pocket universe" or something else. (COMIC: )

The Haemovores evolved on Earth circa 500,000 AD, after the planet was ruined by industrial waste. They lived under the chemical swamps, in the saline solution of the ocean, consuming plankton and sometimes ganging up on a great whale to feast. (PROSE: Goth Opera) Eventually, despite their adaptions, the Haemovores died down to just one, "the Ancient One". Having selected them as one of his "Wolves", Fenric used a time storm to hurl them back in time to 9th century Transylvania, using the Ancient One as the progenitor of a number of vampires who would carry out Fenric's will over the following seventeen centuries, primarily tracking down and recovering the flask in which the spirit of Fenric himself had been imprisoned by the Doctor. In 1943 Northumberland, Fenric was finally freed in a World War II military base, and tried to exact his revenge on mankind by forcing the Ancient One to pollute the seas with a poison developed at the military base. However, having heard Fenric gloat about tricking his Wolves into bootstrap paradoxes as a "game", the Seventh Doctor allowed the Ancient One to realise that carrying out this order would in fact crate the ruined Earth from which the Haemovores had arisen. Refusing to participate in the loop, the Ancient One turned Fenric and destroyed both themself and him. (TV: The Curse of Fenric)

Behind the scenes
In contrast to the implication in The Curse of Fenric that the Ancient One had cancelled out its own existence by refusing to carry out Fenric's orders, History of Mankind according to Doctor Who claimed that the Haemovores were a part of the familiar, linear timeline of the Doctor's universe, placing their existence on a polluted Earth abandoned by humanity between the years 5000 and 10,000. It is further stated that the Haemovores went extinct as the planet's ecological balance was restored, leaving it to be reclaimed by humanity.