According to the book, it is highly possible that this dragon could have been around since the age of the dinosaurs.
Attacking the Escape Ship
After freeing the Wanderer slaves from the Hysteric Tribe, Hiccup Horrendous Haddock III, Camicazi, Fishlegs No-Name, and the Wanders all flee from their ship, The American Dream 2, by using a lifeboat. Unfortunately, while escaping, they accidentally awaken the sleeping Leviathorgan, who chases Hiccup, Camicazi, and Fishlegs back onto the ship and very nearly kills them.
Attacking The American Dream II
The Leviathorgan is briefly repelled from The American Dream II due to a dragon-repelling device made by Norbert the Nutjob. However, just as Norbert threatens to kill Hiccup, who manages to start a rebellion against Norbert, it eventually gets past the repellent and begins to attack the ship, seeking revenge for Hiccup waking it up. Knocking Camicazi and Fishlegs overboard, the massive sea dragon begins to chase Hiccup and Norbert. Hiccup manages to trick Norbert into following him to the top of the ship's mast and electrocuting himself. This also ends up destroying The American Dream II and killing the Leviathorgan in the process.