No one quite knows for sure what the Esophagor is but his monstrous appetite is as clear as day. Many a Neopet has gone missing when they strayed too close to the Esophagor. However, if you take on one of his quests for food he may reward you with something.
He also has a brief appearance in Neoquest II, refusing to let Rohane and his friends through to Edna's Tower. He's the final boss of the fourth chapter, the Haunted Woods.
Completing the Esophagor's quests is the only way to complete a Brain Tree quest.