Wargs are wolves.
They are friends of the goblins. They warn the goblins that the dwarves are up the pine trees on the other side of the Misty Mountain. They prevent the party from escaping and allow the goblins to catch up. All is nearly lost when the trees that the party are in are torched. Luckily the Giant Eagles rescue the party.
(This image is from the 1977 film version of the story)
(This image is from the 1977 film version of the story)