What are the people of Rohan attacked by as they flee to Helm’s Deep? They are attacked by Wargs.
In Peter Jackson’s film The Lord of the Rings: The Two Towers, King Théoden leads the people of Rohan away from Edoras toward the fortress of Helm’s Deep. During the journey, Saruman’s forces launch an ambush using Wargs—large, wolf-like creatures ridden by Orcs.
The attack becomes a major action sequence. Aragorn, Legolas and Gimli help defend the refugees and Rohirrim, while Aragorn is dragged over a cliff during the fight and briefly presumed dead. He later survives and reaches Helm’s Deep, where Rohan prepares for Saruman’s much larger assault.
This detail is associated especially with the film adaptation. Tolkien’s book describes the journey and the Battle of Helm’s Deep differently, without the same Warg-attack sequence involving the refugee column. That book-versus-film distinction is the main source of confusion, but for the movie question, Wargs is the canonical answer.