Physiology and uses
Targs were comparable in form to Terran boars but with spikes on their backs. They were usually dark brown, although some were spotted. Targs were generally regarded as "vicious and destructive" animals. Klingons kept domesticated targs as pets and livestock, and hunted wild targs for sport. The meat of the targ – including the heart – was eaten, while their shoulder fat was used to make var'Hama candles. (TNG: "A Matter Of Honor"; DS9: "You Are Cordially Invited"; VOY: "Day of Honor") Targs produced milk; targ milk could be used as an ingredient in beverages such as the Prakal II mixed drink. (TNG: "In Theory") Wistan gagh was packed in targ blood. (DS9: "Field of Fire")
Klingon wedding ceremonies sometimes involved the ritual sacrifice of a targ. (DS9: "A Time to Stand") As part of the Day of Honor ceremony, the participant had to eat from the heart of a sanctified targ. (VOY: "Day of Honor")
Targ herds could be a collision hazard on Qo'noS; Klingon ground assault vehicles used targ scoops to clear the animals away from their path. (VOY: "Elogium") A behavior typically associated with targ was hiding in dirt. (ENT: "Marauders") The Hamar Mountains were abundant with targ. (DS9: "You Are Cordially Invited")
One of the earliest encounters Humans had with targs was when, in 2151, members of the Enterprise NX-01 crew boarded the IKS Somraw in a rescue attempt. They found the carcasses of three targs, hanging up in the ship's galley, and also discovered three living targs, chained up behind a metallic door in the galley. (ENT: "Sleeping Dogs")
During a conflict between Klingon troops led by Captain Korok and a combined force of colonists from a deuterium mining colony and officers from Enterprise, Korok described the behavior of his opponents when they were trying to evade him and his officers as "hiding in the dirt like targ." (ENT: "Marauders")
Later that year, Enterprise captain Jonathan Archer was provided a bone joint from a targ, served on a metal plate, while enduring imprisonment on Narendra III. Archer decided, based merely on the appearance of the meat, that he disliked Klingon cuisine and Denobulan doctor Phlox concurred that the food did not look appetizing. A while after Phlox advised Archer to eat the targ meat because it seemed to be rich in protein, Archer tried it but almost immediately spat it out, moments later telling Advocate Kolos that it tasted "a little underdone." (ENT: "Judgment")
In 2154, Andorian Commander Shran remarked that he would prefer to live with targs than collaborate with Tellarites, an example of the hatred that existed between the Tellarites and Andorians. (ENT: "United")
Both Martok and Worf owned pet targs at some point in their lives. (DS9: "Strange Bedfellows"; TNG: "Where No One Has Gone Before") Martok once described a targ he had owned during his childhood as "a filthy, mangy beast, but in his bony breast beat the heart of a warrior" and "faithful" to its master but "ever-ready to follow the call of the wild." Martok's wife, Sirella, detested the creature. It escaped into forestland after she "accidentally" left the door of their home open while moving her belongings into the residence following their marriage. Despite Sirella being responsible for the loss of the creature, Martok later told Benjamin Sisko that he would "not trade Sirella for all the targs on Qo'nos". (DS9: "Strange Bedfellows")
In 2364, an illusory replica of Worf's targ was created, from his memories of the animal he had as a pet when he was a child, on board the USS Enterprise-D. The creature appeared when the ship traveled a billion light years away from our galaxy and entered the outer rim of the universe, where the physical and mental world are integrated. As a pet, Tasha Yar asked if that was to him a "kittycat", to which Worf concurred, "yes, I suppose you could call it that." (TNG: "Where No One Has Gone Before")
After Ira Graves' intellect was transferred from Data to the main computer of the USS Enterprise-D, Commander Riker claimed to Data – who had no memory of the events while Graves' mind had possessed him – that he had wrestled with a targ. Extremely puzzled by what had happened, Data wondered if he had won the contest. (TNG: "The Schizoid Man")
In early 2373, Odo told Benjamin Sisko that Sisko would need someone who could turn into Gowron's pet targ to help infiltrate Ty'Gokor, something Odo was unable to do, after Odo had lost his shapeshifting ability several weeks earlier. (DS9: "Apocalypse Rising", "Broken Link")
Quark once sold a herd of targs to an unknown party at some time in or prior to 2369. In 2372, he speculated that Odo had trashed quarters in the form of a targ. In 2373, he believed that by throwing away his bat'leth and kneeling before Thopok, Thopok would "be humiliated and slink away like a scalded targ." (DS9: "The Storyteller", "Crossfire", "Looking for par'Mach in All the Wrong Places")
After Worf was appointed Federation ambassador to the Klingon Empire following the Dominion War, Martok was delighted as he had an ambassador who would go targ hunting with him. (DS9: "What You Leave Behind")
During a musical recital by Harry Kim, The Doctor commented to B'Elanna Torres that Kim's saxophone playing reminded him of a wounded targ. A month later, in 2378, the EMH confessed this act to Kim and apologized for his indelicate behavior. (VOY: "Renaissance Man")
The targ received its name before it was decided what type of animal it was to be. In the revised final draft script of "Where No One Has Gone Before" (dated 30 July 1987), it is noted that the form of creature the targ is was "yet to be determined," though the script goes on to say "it is LARGE."  One early design for a targ was submitted by Rick Sternbach. This was far different from how the creature turned out, however, bearing much more of a reptilian appearance. (Star Trek: The Magazine Volume 2, Issue 12, p. 48) Rob Bowman, who directed "Where No One Has Gone Before", explained, "The Klingon Targ basically went through five different designs. Nobody wanted it to look like anything on Earth, and yet they didn't want it too ridiculous-looking either. Finally, the producers just said, 'You decide.'" (Starlog #136 [page number? • edit])
The word "targ" has been featured in several pronunciation guides in DS9 scripts. These include "Apocalypse Rising" and "Looking for par'Mach in All the Wrong Places", but both guides suggest pronouncing the word exactly as it is spelled.   The word also appears in the pronunciation guide in the script for "Prodigal Daughter", which alternatively instructs that the word is to be pronounced "targh". 
Animal meat eaten by Harrad-Sar in ENT: "Bound" may have been a skinned targ. The meat can be seen on Harrad-Sar's ship, during a scene in which three Orion slave girls dance. The episode's final draft script referred to the meat merely as "some kind of roasted animal." It is also possible that a Klingon monster dog seen in Star Trek III: The Search for Spock and Jackal mastiffs seen in Star Trek VI: The Undiscovered Country were different breeds of targs based on similar appearances and similarity to canine breeds.
According to the reference book Star Trek: The Next Generation Companion (3rd ed., p. 57), the menu at the Café des Artistes in TNG: "We'll Always Have Paris" included "Klingon Targ a la mode" among its delicacies. If true, this set a hidden precedent of targ being consumable that was followed by the introduction of both heart of targ and targ milk on TNG (in "A Matter Of Honor" and "In Theory" respectively).
According to Doug Drexler, Star Trek: Enterprise prop master Craig Binkley lamented that he was struggling to find a food stylist who counted, among their cookery skills, being as creative as to make "flaming Targ testicles too!” This was immediately before Drexler told Binkley about Dorothy Duder, who went on to serve as the series' food stylist. 
In polish language "targ" means "market".
In Star Trek Online, some Klingon NPCs referred to as "Targ Handlers" can summon targs that will attack the player. Additionally, in Klingon missions taking place in the Ketha lowlands, wild targs can be encountered. In one Federation mission, where the player uses a holographic disguise to slip past large numbers of Klingons unnoticed, targs can see through the disguise and attack anyways. Klingon players can also purchase targ pets; however, player-owned targs cannot fight.
During one of the missions in Star Trek: Elite Force II, a male Andorian and a female Klingon can be overheard negotiating for transport with a Human freighter captain in the bar on a Klingon station. The Andorian states he requires a large vessel to which the Human boasts her Wodan-class cargo ship can outhaul even the largest Romulan D'deridex-class starship. When asked what the cargo is, the Andorian replies, "Only myself, ten transport containers of domesticated targ and no questions asked." They then disagree on the fee with the Klingon stating they could purchase their own Bird-of-Prey for the price the Human is asking. Eventually, they agree to pay half of the fee at the time and the second half when they reach their destination. This entire exchange is a reference to the famous cantina scene featuring Obi Wan Kenobi and Luke Skywalker asking Han Solo for transport on the Millennium Falcon in Star Wars: A New Hope.