Short answers, pulled from the story.
The word warg emerged from the collision of Old Norse and Old English languages in the mind of J. R. R. Tolkien. He combined the Old Norse term vargr with the Old English word wearh to create a new creature for his fiction.
Norse mythology provided specific examples such as the jötunn Hyrrokin who arrived at Baldr's funeral riding a wolf on an image stone constructed between 985 and 1035 AD. These mythological figures established a tradition where wolves served as mounts for dangerous humanoid creatures.
A ten-year-old boy named Rayner Unwin wrote a positive review of The Hobbit after reading about wargs which led Stanley Unwin to publish the novel based on his son's enthusiastic feedback. Their presence helped shape the publishing history of modern fantasy literature through a child's perspective.
Gary Gygax introduced wargs in the 1976 supplement Swords and Spells but a lawsuit from Tolkien's estate forced creators to rename them worgs along with other terms. This legal renaming ensured compliance while preserving the core concept of an evil riding wolf.
In A Song of Ice and Fire Wargs are skinchangers who enter the mind of animals such as when Bran Stark controls Hodor and sees what they see through their eyes. These divergent interpretations expand the original Tolkien definition beyond mere evil riders to include magical connection or physical transformation.