Sleuther is a Triple Stryke dragon named by Tuffnut and befriended by Toothless. He was a prisoner in a fighting arena for dragons where he won every battle. But with help from Toothless, Sleuther eventually escaped. Despite his new freedom, Sleuther didn't fully trust the Riders. But overtime, he assisted the Riders in some missions. He even allowed Dagur to ride on his back, forming a bond between the two.