Sleuther

Sleuther is a Triple Stryke trained by Hiccup who first appears in Race to the Edge and then appeared in Dragons: Rise of Berk.

History
Sleuther first appeared in the episode Stryke Out, in which he was the champion of a dragon fighting arena owned by the Dragon Hunters. However, fans got tired of the Triple Stryke's constant quick wins and attendance began to drop. Ryker demanded that the manager find a way to make the fights entertaining again, or else. When Hiccup and Toothless were captured in a Dragon Hunter trap and sent to the fight ring, the manager finally found a way to boost attendance, which was to have Toothless battle the Triple Stryke. After a long battle, Toothless won and was about to kill the Triple Stryke, but noticed the pain and sadness in its eyes and allowed it to live. It then helped Toothless, the Dragon Riders, and the other captured dragons break out of the ring and escape. However, unlike the other dragons, the Triple Stryke had been in captivity for so long, that it didn't know how to be free. However, with encouragement from Toothless, it finally left the ring. Flying back towards Toothless, the Triple Stryke revealed that it wanted to go back to Dragon's Edge with the Riders. Tuffnut named the dragon Sleuther, and they all returned to Dragon's Edge

Trivia

 * For some reason, this dragon is referred to as "Slasher" in Rise of Berk.