Dragon armor, also known as battle armor, has been used by certain dragons, trainers and riders to reinforce dragons scales, as well as making them look more ferocious and terrifying.
Drago Bludvist was the first to place armor on his dragons when he used them to attack a gathering of chiefs on Berk twenty years before his victory on Valka's Mountain.(How to Train Your Dragon 2) Eighteen years later and following their first encounter with the Grimborn brothers' Dragon Hunters, Hiccup and Snotlout made armor for Toothless and Hookfang out of Screaming Death scales.("Night of the Hunters, Part 2") In "Edge of Disaster, Part 1", with Fishlegs' help, Hiccup and Snotlout then made stronger sets of armor for Toothless and Hookfang made of Gronckle Iron.
Following these events, Drago continued to place armor on his dragons. However, following Drago's defeat by Hiccup and Toothless in How to Train Your Dragon 2, his dragons' armor were removed by Gobber and Valka. Only Bonesnarl refused to have his armor removed.
According to Gobber(Legend of the Boneknapper Dragon) and his ancestor, Bork the Bold(Book of Dragons, the Boneknapper wore armor made from bones of dead dragons. Like Gobber's Nemesis, Boneknappers would stop at nothing to complete their armor. When completed, their armor would allow them to unleash their roar and mating call.(Legend of the Boneknapper Dragon)
The Armorwing made it's armor out of the metal it could scavenge and find. Armorwings, such as Bandit, need armor to protect itself from other dragons due to its lack of scales.("Snotlout Gets the Axe") It also used it to intimidate other dragons. Once welded onto itself, the Armorwing's armor could not come apart by any dragon flame such as Monstrous Nightmare, Deadly Nadders, and Night Fury. The only dragon capable of taking parts of its armor were Smothering Smokebreaths.("Snotlout Gets the Axe")
Screaming Death Armor
The Screaming Death is said to have the strongest scales of any dragon. Knowing this, Hiccup and Snotlout then went to its island to make armor for their dragons. In "Night of the Hunters, Part 2", Toothless' and Hookfang's armor were able to hold up against the Dragon Root laced arrows of the Dragon Hunters. The only down-side was that it was heavy.
Gronckle Iron Armor
Fishlegs then helped Hiccup and Snotlout make sets of lighter armor for their dragons, Toothless and Hookfang. At high speeds, Toothless' armor came apart.("Snow Way Out") Despite that setback, Hiccup and Fishlegs were able to perfect armor. However, only Toothless and Hookfang had this armor made for them thus far.
Heather was able to make armor out of Windshear scales as shoulder blades and cover her arms.
Armor for Leopold
In the game, School of Dragons expansion "Rise of Stormheart", Harald Forkbeard requests the player get his Terrible Terror Leopold some armor for protection. The player brings him a fancy helmet for his dragon in exchange for information about his boss, Nikora Stormheart.