Klingon

In a parallel universe, the Klingons were a species from the planet Qo'noS who were the leaders of the Klingon Empire.

Biology
The Klingons were humanoids with ridged foreheads. (COMIC: Assimilation²)

History
During the 24th century, they were allied with the United Federation of Planets. One Klingon, Worf, was a Starfleet officer who served on the USS Enterprise-D in the 2360s. Some Klingons were assimilated by the Borg Collective; at least one Borg of Klingon origin appeared aboard Locutus' cube during the Battle of Wolf 359, another was seen following the fallout of the Borg-Cyberman Alliance. (COMIC: Assimilation²)

Alternate timeline
In an alternate future shown to Captain Jean-Luc Picard by the Eleventh Doctor, the Cybermen, using Borg technology, conquered Qo'noS and converted the entire populace into Cybermen. (COMIC: Assimilation²)

Other references
According to local newspapers that drew Mickey Smith's attention to the UFOs above Deffry Vale High School, the invaders were not Klingons from the TV show Star Trek. (WC: Tardisode 3)

Behind the scenes

 * The Klingons are one of the major races of the Star Trek universe. They have appeared many times in all seven television series and all but two of the films.
 * In the Top Gear "Master of the Universe" segment, a Klingon raced around the Top Gear track against a Dalek, a Cyberman and Colin Baker as the Sixth Doctor (as well as Darth Vader from Star Wars and Ming the Merciless from Flash Gordon).
 * In the Doctor Who television story The God Complex, a woman in Howie Spragg's room on the prison ship which held the Minotaur asks Howie how to say "loser" in the language "K-K-K-Klingon". Though The God Complex doesn't make a direct connection to the race, this is an obvious reference to the language spoken by the Klingons.
 * J. K. Woodward, who worked on the Star Trek crossover Assimilation², released a piece depicting River Song dressed as a Klingon holding a bat'leth.