When Tolkien created The Lord of the Rings and the world that those books inhabit, he created something that was very detailed and realized, from all of the places of the world to the humans and creatures that inhabit them. There is so much in these books that even the most hardcore fans don't know all of the lore, and a lot of people might not know every place or creature by name. While this means that the world of Middle-earth feels real and lived in a lot of the time, it can also make things confusing for more casual fans of the story.
Tolkien came up with a lot of evil creatures specifically - things that would hinder the heroes in their journey and create a major obstacle or danger. These kinds of beings are often referred to as servants of Sauron, and can include Orcs, Uruk-hai, and Goblins, just to name a few. Sauron had some non-humanoid servants as well, some of the most well-known of whom were the Wargs. But where did these wolf-like creatures come from, and why are used by the forces of evil?
History of Wargs
Wargs were an evil breed of large wolves, often thought of as demonic and suggested to have been possessed by evil spirits. In the year 2941 of the Third Age, a group of Wargs planned to meet the Goblins of Goblin-town to organize raids on nearby villages because they wanted to get rid of the Woodmen (groups of men in the North who had taken up residence in the forests of Middle-earth), as well as capture some slaves. As the pack came upon the meeting place near the mountains, they encountered Thorin's company of Dwarves (along with Gandalf and Bilbo Baggins), who had just escaped from the very Goblins the Wargs had come to meet. Gandalf, however, had seen this confrontation coming and suggested that the company climb into the nearby trees, which got them out of the Wargs' reach.
The Wargs believed that the company were spies for the Woodmen, so they surrounded the area in order to prevent them from escaping. Gandalf lit some pinecones on fire and threw them at the Wargs until they were finally driven away. The wolves he had caught with the cones were still on fire and ended up lighting parts of the forest up as well, since it was summer and had not rained there in a while. The Goblins also showed up and started lighting the trees the company was taking refuge in on fire, but Thorin's crew was luckily saved by the Great Eagles just in time.
The Wargs were next seen after the news of the dragon Smaug's death had circulated, and both the Wargs and Goblins tried to claim the Lonely Mountain as their own. However, they were stopped by the legions of Elves, Dwarves, Lake-men, and Great Eagles during the Battle of the Five Armies. This, of course, all takes place during the events of The Hobbit.
Much later, in 3019 of the Third Age, the Fellowship of the Ring was attacked by Wargs, sent by Sauron after they could not pass through the mountain peak Caradhras. They fended the wolves off initially, but the Wargs returned again with reinforcements. This attack also failed, and by morning, their carcasses had disappeared. This is the beginning of their appearances in The Lord of the Rings.
Wargs in the Lord of the Rings movies
Wargs appear many times throughout the course of both The Lord of the Rings and The Hobbit. In The Lord of the Rings, their first appearance is in The Two Towers when Saruman sends a group of Orcs riding on the backs of Wargs to attack the people of Rohan as they make their way to Helm's Deep. The leader, Sharku, battles Aragorn and ends up sending him over the edge of the cliff with his Warg, whereupon Aragorn is believed to be dead by the rest of the Fellowship for a time after this. In The Return of the King film, multiple Wargs are seen storming the city with their riders during the Siege of Gondor. The Wargs in Lord of the Rings resemble hyenas, and have short muzzles with huge fangs, with relatively small eyes set on either side of their heads.
In The Hobbit trilogy of movies, the Wargs play a more significant role as the minions of Azog, the White Orc (one of the main antagonists in the story), who himself rides a Warg with a white coat. The Wargs in The Hobbit are called Gundabad Wards, bred by the Orcs of Mount Gundabad. This breed of Warg is faster and more deadly than the others, which makes it more of a threat. Thorin and company are stalked by some Wargs that eventually reveal themselves at the Trollshaws, and though Radagast offers to distract them by veering off course with his sled pulled by Rhosgobel Rabbits, the Warg-riders chase the Company anyway until they are ambushed by Elves. There are also Warg-riders who are part of Azog's army in the final battle.