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 particularly large and evil kind of wolf.
In Old Norse, vargr is derived from the reconstructed Proto-Germanic *wargaz, ultimately derived from the reconstructed Proto-Indo-European (PIE) root *werg̑ʰ- "destroy". In Beowulf, Grendel's mother is described (line 1514) as a grund-wyrgen, which may be translated as "cursed creature of the depths", "bottom-dwelling monster" etc.
vargr (compare modern Swedish varg "wolf") has arisen as a noa-word for úlfr, the normal Old Norse term for "gray wolf" which is related to similar words in other Indo-European languages and is derived from the reconstructed Proto-Indo-European noun *wĺ̥kʷos ("wolf") and, probably also, the PIE adjective *wl̥kʷós "dangerous". These words comprise Proto-Tocharian *wä́lkʷë, Proto-Italic *lukʷos, Proto-Balto-Slavic *wilkas, Old Albanian ulk, Greek *lýkos, Proto-Iranian *verk Sanskrit vṛka, Avestan vehrka, Mazandarani varg, Zazaki verg, Old Persian varka- and Persian gorg, among others, all meaning "wolf". Despite their superficial similarity to Old Norse vargr, the various Indo-Iranian forms are not related to it but derive from *wĺ̥kʷos.
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 Hyrrokkin arrived on a wolf.
Taken from the Old English warg, the wargs or wild wolves are a race of lupines 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, and are consciously in league with the Orcs.
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, hunting for food". Wargs and their riders also appear in The Two Towers.
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, they do seem to have a conventional 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. Wargs also appear in Peter Jackson's The Hobbit trilogy but are a more wolf-like breed exclusive to Gundabad compared to the wargs used by the Orcs of Isengard.
In The Lord of the Rings Online wargs are a playable race on the antagonist side in the game's PvP area. It is widely considered that Thegreatblackwarg, or "Hejazia" as he is formerly known, is the greatest of the PvP Wargs.
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: