Xenomorph

The Xenomorph, (PROSE: Genius Loci, So Vile a Sin, et al.) also known as the "life-spores" to Agent A, were an aspect of the Enemy. (PROSE: The Annotated Autopsy of Agent A)

History
Initially, the "life-spores" (a moniker used by Agent A) were just another danger to space travellers, when they infected the crew of the Narcissus, (whom had discovered the "life-spores" inside a ship of a lesser species on an asteroid) infecting and killing the crewmember James Watt. Soon, the logic of their presence began to affect the Spiral Politic, so that the asteroid the race was found on inconceivably became a colony. One survivor remained, Captain Jennifer Alistoun, who had more fights against the species, both adapting between every encounter. Suddenly, another warlike speices, who had no interaction with the "life-spores", began referencing the "life-spores" (who were now one with the Enemy) in their culture. The Great Houses began to notice the pressing threat of the Enemy's new identity, who had been retroactively inserting themselves into the past, following the passage of events the lead to the Narcissus, finding themselves in the third quarter of the 20th century on Earth.

was an alien species, which the Sixth Doctor once observed in Crozier's laboratory. (TV: Mindwarp)

Several students once used Xenomorph resin to decorate the surfaces of their block of flats, to give it the appearance of, as Simon Frederson put it, "a knobbly, black, and unpleasantly organic shapes and orifices" and " very ugly and retro. (PROSE: So Vile a Sin)

Other references
Heidi Maputo once used the turn of phrase, "[when] the Xenomorph [hits] the ventilation shaft". (PROSE: Genius Loci)

K9 Mark 2 once recommended "rapid movement in the opposite direction of the xenomorph". (TV: Liberation)

Behind the scenes
The Xenomorph was the titular alien antagonist from Alien and Aliens, however, there has yet to be any narrative connection to suggest show that the Xenomorph is from the N-Space equivalent of the Alien franchise; so for all intents and purpose, the Xenomorph are a genuine species that exists within the Doctor Who universe.

The Xenomorph seen within Mindwarp was that of the Chestburster stage of the Xenomorph Life-Cycle.