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In Norse mythology, a vargr (pl. vargar; often anglicised as warg or varg) is a wolf and in particular refers to the wolf Fenrir and his sons Sköll and Hati. Based on this, J. R. R. Tolkien in his fiction used the Old English form warg (other O.E. forms being wearg and wearh) 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 is related to proto-Iranian *verk "wolf", Avestan vehrka, Mazandarani varg, Zazaki verg, Old Persian varka-, Persian gorg etc. In line 1514 of Beowulf, Grendel's mother is described as a grund-wyrgen or "warg of the depths."
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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 Hyrrokkin 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 goblins or 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 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. They are here distinguished from ordinary wolves "looking for food."
In the Rankin-Bass adaptation of The Hobbit, they are portrayed as larger than average wolves with ominously glowing eyes. Although Tolkien never provided 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."
Peter Jackson's The Lord of the Rings film trilogy depicts wargs as hyena-like, or even like the extinct, heavier-bodied hyaenodon rather than wolf-like, due to it looking more powerful. Wargs also appear in Peter Jackson's The Hobbit trilogy but are referred as Gundabad Wargs and appear more wolf-like than the Wargs used by the Orcs of Isengard.
Subsequent appearances of wargs in popular culture often owe much to Tolkien. Similar to Tolkien's works, they are often depicted as evil, intelligent wolves that speak their own language, and are often allied with goblin tribes: