Wan Shi Tong

Wan Shi Tong is the spirit of knowledge — an ancient and powerful being who knows more than any human walking the earth. Although his regular shape is that of an immense black-and-white owl, an animal commonly associated with wisdom in Western thought, he can lengthen his body to that of a wyvern-like creature when angered.

History
Long ago Wan Shi Tong brought his library from the spirit world to the physical world. Together with his band of knowledge seeking foxes they collected knowledge from all over the world. He put it all on display in his library for humanity to read and hopefully better itself. Although he left his great collection of knowledge available to the mortal world for many years, he is recently rather angry at humankind; he apparently sees knowledge as something good purely for its own sake, and in recent years he is known to have been visited by humans only twice — once by Zhao (who was then a young lieutenant) and once by Aang and his friends — in both cases because the visitors were hoping to find secrets to destroying their enemies (the Water Tribes, and Fire Nation, respectively). After Sokka breaks a vow made to Wan Shi Tong that no knowledge found in the library will be used for violence, Wan Shi Tong pulls the library back into the Spirit World by sinking the library, attacking Aang and the others. He was eventually knocked out by Sokka who then, along with Aang and Katara, escaped from the sinking library.

Trivia

 * "Wan Shi Tong" (萬事通) is a phrase in Chinese that means "he who knows 10,000 things", which is what he had called himself in "The Library".
 * Wan Shi Tong's two forms are both based on animals used as symbols for knowledge in the real world, the owl and the serpent.
 * When Wan Shi Tong first appears in "The Library", he briefly looks like the spirit "No-Face" from Miyazaki's "Spirited Away".
 * Wan Shi Tong's alternate form seems to somewhat bear a resemblance to the modern design of dragons.
 * Wan Shi Tong is voiced by Hector Elizondo, who plays Joseph in the Princess Diaries and Bane in Batman: Mystery of the Batwoman.
 * Wan Shi Tong only appears in the episode "The Library" but is mentioned in a few other episodes.