In Norse mythology, a warg is a wolf and in particular refers to the wolf Fenrir and his sons Skoll and Hati. Based on this, J. R. R. Tolkien in his fiction used the Old English form warg to refer to a wolf-like creature of a particularly evil kind.
In Old Norse, vargr is a term for "wolf" (ulfr). The Proto-Germanic *wargaz meant "strangler" (see modern German würgen), and hence "evildoer, criminal, outcast." Varg is still the modern Swedish word for "wolf." Also cognate is Old English warg "large wolf". In Dutch wolverines are sometimes called Warg, although the name Veelvraat is more commonly used.
Heidrek knows the answer is the Sun, explaining,
Wolves also served as mounts for more or less dangerous humanoid creatures. For instance, Gunnr's horse was a kenning for "wolf" on the Rök Runestone, in the Lay of Hyndla, the völva (witch) Hyndla rides a wolf, and to Baldr's funeral, the giantess Hyrrokin arrived on a wolf.
Taken from the Old English warg, the wargs or wild wolves are a race of fictional wolf creatures in J. R. R. Tolkien's books about Middle-earth. They are usually in league with the Orcs whom they permitted to ride on their backs into battle. It is probable that they are descended from Draugluin's werewolves, or of the wolf-hounds of the line of Carcharoth of the First Age. They are portrayed as somewhat intelligent, with a language of sorts, and are consciously in league with the Orcs, rather than wild animals the Orcs have tamed.
The concept of wolf-riding Orcs first appears in The Tale of Tinúviel, an early version of the story of Beren and Lúthien written in the 1920s, posthumously published as part of The History of Middle-earth.
In The Hobbit, the Wargs appear twice, once by working with Orcs (called goblins in the book), in hunting Bilbo Baggins, Gandalf, and the dwarves just east of the Misty Mountains, and once at the Battle of Five Armies.
In The Lord of the Rings, they are most prominently mentioned in the middle of The Fellowship of the Ring, where a band of Wargs, unaccompanied by Orcs, attacks the Fellowship in Eregion. During the War of the Ring in T.A. 3018–19, wolves prowled outside the walls of Bree.
In the Rankin-Bass adaptation of The Hobbit, they are portrayed as larger than average wolves with ominously glowing eyes. Although Tolkien never gave a fully complete description of the Wargs (he simply noted that they were demonic wolves), they do seem to have a regular wolf-appearance in both The Hobbit and The Lord of the Rings, and they are regularly called "wolves."
In Peter Jackson's Lord of the Rings movie trilogy, Wargs appear to be more like giant spotted hyenas or the extinct Sarkastodon rather than wolves. Jackson explained that the hyena design was chosen due to it looking more powerful. Guillermo del Toro, who is set to direct The Hobbit films stated that the wargs would be redesigned: "My belief on the 'Wargs' issue is that the classical incarnation of the demonic wolf in Nordic mythology is not a hyena-shaped creature. It is a wolf. The archetype is a wolf, so we're going to go back to the slender, archetypical wolf that is, I think, the inspiration for Tolkien."
Subsequent appearances of the creatures in popular culture often owe much to Tolkien. In the Dungeons & Dragons roleplaying game, wargs appear as minor enemies. Similar to Tolkien's works, they are depicted as evil, intelligent wolves that speak their own language, and are often allied with goblin tribes. The large wolf-like enemies in the Castlevania video game series are also called Wargs.
In the A Song of Ice and Fire fantasy novel series by George R. R. Martin, wargs are people who can form a telepathic-empathic bond with an animal. While this link is active, the human perceives and experiences what the wolf perceives/experiences.
In the World of Warcraft roleplaying game, worgen are intelligent, wolf-like, forest-dwelling creatures similar in look to Tolkien's wargs; they form an important part of one of the game's early dungeons. Worgs are mainly located in the undead inhabited areas of the game. Many worgs (mainly in the plaguelands) are also undead, with the general appearance but minor differences such as rotting flesh or bones showing in their abdomen. Orcs also tame certain worgs as sturdy mounts, similarly to Tolkien's idea.
Also in Warcraft Orcs and Humans as well as Warcraft 3 orcs appear riding worgs
Wargs appear in the PC game saga Gothic as well.
Larry Correia's Monster Hunter International features giant wolf-like creatures used by orcs as mounts. However, it should be noted that the orcs of the MHI world are good and fight against the evil monsters. It is unclear whether these warg-like mounts are intelligent.