Xenomorph

Xenomorphs were a parasitic species encountered by humanity from the 21st century onwards. They evidently had several "stages" in their life-cycle.

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

The Sixth Doctor once observed, in apparent disgust, what appeared to be an infant, "chestburster" Xenomorph in Crozier's laboratory, (TV: Mindwarp) and in 2012, Van Statten's Vault had what seemed to be a Ovomorph on display. (TV: Dalek)

Agent A once wrote a report about creatures born from "life-spores". These life-spores were believed by A to either be the Enemy or an aspect thereof; they could grow into creatures which would burst out of the chests of humans. (PROSE: The Annotated Autopsy of Agent A)

Heidi Maputo once used the saying "when the xenomorph hits the ventilation shafts". (PROSE: Genius Loci)

Notably, K9 Mark 2 used the term "xenomorph" more loosely to apply to other alien beings, and, accordingly, once recommended "rapid movement in the opposite direction of the xenomorph" in reference to a Jixen warrior. (TV: Liberation)

Behind the scenes
In the real world, the science-fiction/horror films Alien and Aliens, being part of the wider Alien franchise, featured Xenomorphs, although nothing in valid DWU sources has drawn a link between the scattered allusions to Xenomorphs as real beings, and the occasional contextless nods to Alien and Aliens as in-universe movies. Some sources have indirectly referenced the relationship between the Xenomorphs and the Alien series, being: Bernice Summerfield's comparison of Ace's medical breathing apparatus and a facehugger in PROSE: The Left-Handed Hummingbird; Toshiko Sato's recollection of the scene where the "thing" bursts out of the chest of John Hurt in TV: Greeks Bearing Gifts; and when the Dream crabs were likened to Facehuggers in TV: Last Christmas.

Although the DWU and Alien have never had an official crossover acknowledged by the Alien side of the equation, an actual Chestburster prop was clearly visible in TV: Mindwarp, hinting at a shared universe as early as 1986. The Xenomorphs and the Weyland-Yutani Corporation continued to have cameos in television stories and novels alike for years to come.

A more narratively relevant crossover came in the form of The Annotated Autopsy of Agent A, which, although it did not mention the name "Xenomorph" specifically, established that the Xenomorphs were a fictional depiction of the Life-spores after the latter species became the Enemy and inserted themselves into the meta-flow of Earth's popular culture. The Life-spores were described in such a way that they were immediately recognisable to anyone familiar enough with the Alien franchise, whilst also have notable differences despite that. Interestingly, this meant that despite The Annotated Autopsy of Agent A featuring a species that was ostensibly the Xenomorphs, it was ironically more in continuity with stories that treated creaturs from the Alien series as wholly fictional.

Other matters
In the charity anthology The Curse of Fanfic!, in the story Alien Encounter by John Peel, the Xenomorphs from Alien crossed over with Fireball XL5.