"Hur'q" was a Klingon word meaning "outsider," and was the name given by the Klingons to a species from the Gamma Quadrant which invaded and plundered Qo'noS in the 14th century. Among the most valuable artifacts stolen by the Hur'q was the Sword of Kahless, which was only recovered a thousand years later, in 2372. (DS9: "The Sword of Kahless")
By 2372, the Hur'q were extinct, and their stolen artifacts remained unrecovered. In that year, a Vulcan geological survey team mining bakrinium on an uncharted planet in the Gamma Quadrant accidentally uncovered ancient ruins which belonged to the Hur'q. This led to a sequence of events in which Dahar master Kor, Lieutenant Commander Worf, and Lieutenant Commander Jadzia Dax were able to finally recover the long-lost Sword of Kahless, although they eventually decided not to return it to the Empire. (DS9: "The Sword of Kahless")
This species was only mentioned in dialogue.
The Hur'q were also featured in the video game Star Trek: Invasion, where they came back through a wormhole and had to be stopped from entering this area of the Galaxy to prevent them from taking it over. They were insectoid scavengers, whose physiology resembled that of army ants and the armor of the samurai.
According to the manual for Star Trek: Klingon Academy, the Klingons acquired warp drive from the Hur'q, by capturing and reverse-engineering some of their ships when they resisted their incursions. This explained how the Klingons, despite not being focused on technological research, managed to make the societal jump to interstellar travel.
The Hur'q play a major role in the storyline of Star Trek Online. The Tzenkethi, who referred to the Hur'q as "Drantzuli", embarked on a crusade against the creatures using protomatter weapons to "purify" any planets where their eggs were found, which brought them into conflict with the Alliance (the Federation, the Klingon Empire, the Romulan Republic, and their assorted allies). When the Alliance forces move to defend Bajor from the Tzenkethi, the Hur'q emerge and nearly destroy Deep Space 9 before they are fought off by a Dominion fleet led by Odo.
The "Victory is Life" expansion reveals that over two thousand years before, the Hur'q (or the "Burrowers" as they were called by the Dominion) had initially been an intelligent, peaceful insectoid race from the planet Havas-Kul. The Female Changeling sought to turn the pacifist Hur'q into soldiers of the Dominion (prior to the creation of the Jem'Hadar) by removing a crucial fungus, a precursor to ketracel-white, from their homeworld's ecosystem as a means to control them. This instead drove the Hur'q to madness, focused solely on consuming life. The Dominion still used the Hur'q against their enemies by luring the swarms with special signal beacons; by the game's setting in 2409-10, however, the Hur'q had become too wild, and brought the Dominion to the brink of collapse. Out of desperation, Odo secretly planted Hur'q lures on Bajor and other worlds in order to convince the Alpha Quadrant powers to join him in combating the Hur'q; at the end of the initial campaign, another lure was placed on the Founders' homeworld of Empersa by Elim Garak as insurance against the Female Changeling. The efforts of Dr. Julian Bashir and the sacrifice of the Jem'Hadar Elder Dukan'Rex manage to restore the Hur'q's cognitive function and end the conflict before they destroy the Great Link. The Alliance (which now includes the Dominion) sets up a neutral zone around Havas-Kul and organizes relief efforts to help the Hur'q rebuild their lost civilization.