Xenomorph

Xenomorphs were a parasitic species encountered by humanity from the 21st century onwards.

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 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". (TV: Liberation)

Behind the scenes
In the real world, the science fiction 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. The closest any source has got, was in TV: Last Christmas, when a Facehugger was identified as being from the films, as well as in TV: Greeks Bearing Gifts, when Toshiko Sato mentions the scene in Alien where the "thing" bursts out of John Hurt. (Additionally, the mention of Facehuggers in PROSE: The Left-Handed Hummingbird is open to interpretation if they're fictional or not.)

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", described the history of the life-spore creatures in sufficient detail that any reader familiar with the other franchise would recognise them as Xenomorphs, as readily as viewers were expected to identify the Mindwarp Chestburster or the egg in Dalek.

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.