Festus

"You named the dragon Festus, Leo? Do you know in Latin, Festus means 'happy'? You want us to save the world on Happy the Dragon?"

- Jason Grace

Festus (meaning "Happy" in Latin) was an automaton dragon created by the old Hephaestus cabin, with the blessing of their father, many years before the series to defend Camp Half-Blood but goes haywire after Beckendorf's death.

History
Sometime before the events of the books, Festus was created and activated inside Bunker 9 by the Hephaestus campers and a blessing from the fire god. A set of wings was revealed inside the bunker in The Lost Hero; it's unknown whether they were originally for the dragon and, if they were, why they were never put on him. Also, before his creation, a set of blueprints was made for a Greek trireme lit by flames, and Festus' head was planted in the bow of the ship.

Festus was created to defend the camp from monsters before Thalia was turned into a tree by her father to save her life. However, about fifteen years before Percy's arrival at camp, he malfunctioned and ran off into the woods, where he eventually broke down. His body was somehow buried in the ground and he was lost for a long time.

The Bronze Dragon
It first appears as the unnamed Bronze Dragon. During a game of Capture the Flag, its head is found by the Myrmekes and the dragon is reactivated by Percy, Annabeth, and Silena to help them rescue Beckendorf from getting eaten by the Myrmekes. Beckendorf activates the dragon's "Attack mode" and it ends up running around trying to kill anything, including the demigods. He is deactivated by Beckendorf with Percy's help afterwards due to the fact that it is unstable and dangerous. It is fixed by the Hephaestus cabin to protect the camp sometime after this.

The Lost Hero
Following Hephaestus' cabin being supposedly cursed after Beckendorf's death, the dragon goes haywire and hides in the forest. After Leo catches it in a trap, he manages to fix it, giving it wings and naming it "Festus". Festus then helps Leo, Piper, and Jason on their quest by taking them across the country. It takes them first to Quebec and then to Detroit, where it is knocked out of the sky by Khione, who had also damaged its control disk earlier. After Leo fixes it, it takes them to Chicago and saves them from Medea's sun dragons. It makes its final flight to Omaha, where it malfunctions once again due to Khione's damage. Leo is able to temporarily reboot him before attempting to crash land on a massive mansion's lawn. However, the mansion turns out to belong to King Midas, and a set of lasers cut Festus to shreds. It then uses Morse Code to give Leo his last wishes, stating that "everything can be reused", just as Hephestus had told Leo earlier. It is unknown how Festus knew what the god had said. Leo then asks his father to transport Festus' head back to Bunker 9.

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 the boat would eventually be determined by Leo as the same as the blueprints of what he would call the Argo II. Leo announces to his cabin-mates and a few other campers that the Hephaestus cabin would build the boat, using the head of Festus as the figurehead, to voyage to the Greece mainland and to stop the Giants from destroying the original Mount Olympus, thus destroying the "roots of the Gods."

The Son of Neptune
He is briefly mentioned by Percy when he sees the Argo II, and says the masthead looks "oddly familiar," referring to the time he caught Festus in The Bronze Dragon.

The Mark of Athena
He is the masthead of the Argo II. Festus can also communicate with Leo using a series of sounds similar to Morse code. Despite him being only a head, Festus has a number of useful abilities, such as tracking Celestial Bronze.

Appearance
Festus is described as having a perfect mix of bronze and gold scales, giant sphere rubies for eyes that can light up like high-powered searchlights, bronze wings, and drill-like razor sharp teeth. He is close to 50 tons in weight but can still easily fly with passengers.

Trivia

 * 'Festus' is Latin for 'Happy'. Jason notices this, and jokingly calls him "Happy the Dragon" in The Lost Hero.
 * The Brazilian translation has Jason calling him "Happy Dragon", thus neither using "Happy" as his name nor using "Dragon" as a title.
 * Leo has a safety whistle, summoned from his tool belt, which Festus will come to when it is blown.
 * So far, Festus is the only character who in Percy Jackson and the Olympians appears only in the Demigod Files, but becomes important in The Heroes of Olympus.
 * When asked if Rick Riordan would rather have Blackjack or Festus, he replied: "At the risk of hurting Blackjack's feelings, Festus for sure."