Xenomorph

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

Biology
Xenomorphs went through a life-cycle, where they grew as egg-like "ovomorphs", (TV: Dalek) before progressing to a "chestbuster" state (TV: Mindwarp) and then they would mature into their final, slender black form. (COMIC: Secret Agent Man)

Xenomorphs secreted a resin, which when hardened, looked "knobbly, black, and [had] unpleasantly organic shapes and orifices" and was "very ugly and retro", according to Simon Frederson. (PROSE: So Vile a Sin)

History
In 2012, deep within Henry van Statten's Vault, part of his collection was a Ovomorph. When the Ninth Doctor and Rose Tyler materialised the TARDIS in the Vault and exited the box, they passed the Ovomorph, which was to their right. The duo — and Adam Mitchell — passed it by again when they left the Vault after the self-annihilation of the Dalek nicknamed "Metaltron". (TV: Dalek)

In 2379, in Crozier's laboratory, the Sixth Doctor once observed what appeared to be an infant, "chestburster" Xenomorph, within a look of revulsion. (TV: Mindwarp)

In 2562, Heidi Maputo used the saying "when the xenomorph hits the ventilation shafts". (PROSE: Genius Loci)

In 2982, a group of students used Xenomorph resin to decorate the surfaces of their block of flats, which disgusted Simon Frederson. (PROSE: So Vile a Sin)

In the 54th century, a Xenomorph wearing a tuxedo was present on the Stardreamer II; Captain Jack Harkness sprinted past it. (COMIC: Secret Agent Man)

During the War in Heaven, 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 and had an increasingly complex retroactive history. (PROSE: The Annotated Autopsy of Agent A)

Other references
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)

As fiction
Facehuggers, on a couple of occasions, were identified as a fictional alien lifeform originating from the horror movie simply titled Alien, (TV: Greeks Bearing Gifts, Last Christmas) which to Bernice Summerfield, a native of either the 25th century (PROSE: Falls the Shadow) or the 26th century, (AUDIO: Just War) was "ancient". (PROSE: The Left-Handed Hummingbird)

In 1994, in Mexico City after Ace was hospitalised, Bernice Summerfield visited her and though that the respirator attached to Ace clung to her like a facehugger. (PROSE: The Left-Handed Hummingbird)

In the mid-2000s, Toshiko Sato compared the hole in the skeleton of a 19th century soldier to "that bit in Alien where that thing bursts out of John Hurt". (TV: Greeks Bearing Gifts)

In the mid-2010s, Professor Albert Smithe noted a resemblance between the dream crabs' method of consuming their victims' brains, by latching onto their face, and the facehuggers. (TV: Last Christmas)

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 creatures 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.