Festus

Festus was an automaton created to defend the camp by the Hephaestus cabin, with the blessing of their father, many years before the series. It was created before Thalia's tree gave the camp magical borders. The dragon kept the camp safe for over a decade and then it ran away because it was starting to malfunction. The dragon was friends with Beckendorf and Leo, the latter of whom named him Festus.

The Demigod Files
During a game of Capture the Flag, its head was found by the Myrmekes and the dragon was reactivated by Percy, Annabeth, and Silena to help them rescue Beckendorf from getting eaten by the Myrmekes. He was deactivated by Beckendorf afterwards, due to the fact that it was unstable and dangerous.

The Lost Hero
Following Hephaestus' cabin being cursed after Beckendorf's death, Festus goes haywire and hides himself in the forest. Leo manages to catch him in a trap, fix him and gives him wings. Festus then helps Leo, Piper and Jason on their quest by taking them to Quebec and Detroit. Festus also takes them to Omaha, but he was he was destroyed in the process due to Midas' security systems.

Later on, it is revealed that the head of Festus was in the drawing of a boat that Leo created when he was a child, and this boat would eventually be determined by Leo as the same as the blueprints of what he would call the Argo II. Leo announced to his cabin-mates and a few others that the Hephaestus cabin would build this boat, using the head of Festus as the figurehead, to voyage to the Greek mainland and to stop the Giants from destroying the original Mount Olympus, and thus destroying the "roots of the Gods".

Trivia

 * 'Festus' is Latin for 'Happy'. Jason notices this, and jokingly calls him "Happy the Dragon".