The Fell Wargs were a vile race of Wolves. While the lesser or common Wargs were just greater Wolves the Fell Wargs were intelligent beasts possessed by evil Spirits, therefore Fell Wargs were also considered (lesser) Werewolves ("Gaurwaith").
They were known under various monikers True Wargs, Ghost-Wolves, Gûlscaru, Shadow Wargs, Dire-Wargs, Shadowmaws, Warg-Wights and Deg-lic. The common Wargs or Mountain Wargs and their northern Frost Warg Cousins were just their lesser descendants and the common steed bred by the Orcs in their caves.The Fell Wargs were however a race that lived under it's own rule and only occasionally joined their Orc-comrades as equal allies.The Warg's Ruler was the semi-legendary Warg-King.
The Warg-pack the Fellowship of the Ring had to fight in Eregion were greater Wargs.Their bodies diminished after they had been slain and Gandalf addressed them as Gaurwaith and stated that these were not normal Wolves.
The Wargs might correspond to the "megafaunal wolf" or "cave wolf".