Festus

Festus the Bronze dragon was an automaton created to defend the camp by the Hephaestus cabin with the blessing of their father. It was created before Thalia's tree gave the camp magical borders to protect the camp. The dragon kept the camp safe for over a decade and then it ran away because it was starting to malfunction.

The Demigod Files
Its head was found by the Myrmekes and the dragon was reactivated by Percy, Annabeth, and Silena to help them rescue Charles Beckendorf from the Myrmekes. It was deactivated after that, due to the fact that it was unstable and dangerous.

The Lost Hero
The cover of the new series shows Jason, Piper, and Leo sitting on the dragon, now named Festus which means happy. Leo fixes him and gives him wings. They take him to Quebec and then Detroit and then to Omaha were he was destroyed due to Khione. It is revealed that his head will be the mast in Leo's flying boat, the Argo II.