The Dragon Horn is a small, black horn that is small enough to be held in a man's hand. It also has a resemblance to a dragon's talon. It has a hollow inside, allowing one to blow into it.
The Dragon Horn allows the user to summon a dragon and control it. However, according to D.R., not just anyone can use the horn, and it took him three years to master it in order to control a dragon.
Cyrano stole the Dragon Horn from a castle after killing all the soldiers guarding it ten years before meeting Ryuma, and the incident faded into darkness. Three years later, he and his partner D.R. used it to attack and rob Flare's hometown.
Seven years later, D.R. used the horn to scare away the civilians of a town that both Ryuma and Flare were in so they can loot the place, and D.R. broke a fake horn so that no one could supposedly control the dragon again, while falsely claiming that Ryuma forced him to do it. Eventually, when the truth came out and Cyrano was killed by Ryuma, D.R. threatened the samurai with the fact that he is the only one who could use the horn to control the dragon, but Ryuma ignored his warning and killed D.R. as well, leaving the horn lying next to the conman's corpse.