Great Protector is a female Eruptodon, a Boulder Class dragon species. Great Protector lived inside a volcano on Caldera Cay and prevented said volcano from erupting, for which it is worshiped by the Defenders of the Wing. However, Great Protector was captured by the Dragon Hunters when Queen Mala was interrogating Hiccup Horrendous Haddock III and the rest of the Dragon Riders.
Becoming the "Great Protector"
Great Protector lived on the island, Caldera Cay. At some unknown time, Great Protector became the guardian of the human tribe that inhabited Caldera Cay, the Defenders of the Wing. She protected the Defenders of the Wing by preventing the volcano on the island form erupting, which would destroy their village, doing this by eating the lava.
One day, in the episode "Defenders of the Wing, Part 1", after being led by a supposed truce with the Dragon Hunters, the Dragon Riders arrived on Caldera Cay. When they got there, the riders saw a village, but were kidnapped by the Defenders of the Wing. The Defenders of the Wing believed the riders to be Dragon Hunter spies, but eventually Hiccup was able to convince their queen, Mala of the truth. However, the truce was all a ruse by Viggo Grimborn to lead the riders to Caldera Cay for use as a distraction, while they themselves went into the volcano and knocked out the Great Protector with Dragon Root arrows and kidnapped it. When Mala and Hiccup noticed that the volcano had excess lava spilling out, they walked to it and discovered the ordeal.
The Eruptodon appeared when Viggo and Ryker where visiting it in the cell bay at the Dragon Hunter ship. When Viggo's armanda arrived at the Edge they released the dragon to the mountains of the Edge. When the Eruptodon would start eating the volcano, at some point it will erupt and sink the island. However, when Hiccup and the riders, along with Mala came to the Edge the Hunters attacked. Later, they overpowered the Hunters and rescued the Edge by making a lava trail back to Caldera Cay, with the help of the island's wild Gronckles living on the Edge.
The Eruptodon briefly appeared when Snotlout was tested. Throk signaled the Protecter to fire lava blasts in order to make the rocks for the first trial.
The Protecter laid an egg for an offspring to train it to become the next Great Protector. However the Eruptodon was too weak to fly the egg to the lava chamber for the ritual of birth. So Mala, Hiccup, Fishlegs, and Meatlug flow into the lava chamber and with the help of the fire terrors the ritual ended to be a success. The Protector later gained enough strength to save Throk from the lava.
When Ryker and the Shellfire attacked the island the Eruptodon gave the riders permission to take her offspring away from the Hunters, to keep it safe.