Draco Aionius

The Draco Aionius is a monster that attacked Camp Half-Blood while Percy, Annabeth, Clarisse, Tyson, and Grover searched for the Golden Fleece due to the failing of Thalias tree.

The Sea of Monsters
The Draco Aionius attacked the camp during the Quest for the Golden Fleece. It burned down the arts and crafts cabin, and filled up the Big House with the wounded. The Apollo campers had to put in overtime to heal the wounded as a result.

Triva

 * All Percy could guess about the Draco Aionius was that it Latin for "really-big-lizard-with-breath-that-blows-stuff-up."
 * The Draco Aionius was the only monster mentioned that attacked the camp while the quest was going on.
 * The book never mentioned the Draco Aionius being slain.