Serpent

"The Serpent" is a fairly large aquatic creature that roams about in the seas in the Earth Kingdom, and is most notiably known to live near the strip of land surround by water called The Serpent's Pass. Aang, Katara, Sokka, Toph and Suki first encountered this creature when traveling with Than, Ying, and Than's sister along the Serpent's Pass after their possesions and passports were stolen in Full Moon Bay. They encountered The Serpent again while travelling through the Earth Kingdom aboard a stolen Fire Nation Ship. It started to attack the gang's ship, but turned it's focus on the Fire Nation ship that was chasing them when they fired a fireball at it. The Serpent is teal green with purple lines across his body and has bright red eyes with a long mouth, a long tongue, with large fins protrudes from the sides of it's head and back.

Trivia
The Serpent was featured in the pilot episode of Avatar. He rised from the ocean and stood before Prince Zuko's ship. Later on in the episode, The Serpent's burned head is seen being brought into the fort where Zuko is staying.