Unagi

The unagi is a gigantic carnivorous eel that is known to prey on almost any animal in the water, but feeds almost exclusively on elephant koi. It is capable of spewing out large jets of water at a very high velocity, almost like a geyser. The only known unagi lurked in the bay of Kyoshi Island and was seen when Team Avatar came to the island. The unagi living near Kyoshi Island was later mentioned by Sokka and Suki while crossing the Serpent's Pass.

Anatomy
The unagi's body seems to be several hundred feet in length. It has smooth, dark brown skin, without any scales. Pointed finlets line the back of its body from head to tail. A large dorsal fin protrudes from the top of its head, and appears to be made of approximately seven cartilage structures with connective webbing. The unagi can expand and contract this fin. Below that, on either side of its head, are four bright yellow stripes, possibly its gills. The unagi has bright green eyes and a mouth with sixteen pairs of sharp, interlocking teeth. The creature has two very long barbels on either side of its mouth. Inside its body, below its head, are glands that it uses to blast water at its prey.

Connection
The unagi appears to be based on a giant freshwater eel.

Behavior
The unagi is a very dangerous creature. Being a carnivore, it will eat almost anything unlucky enough to stray into its path, though elephant koi are its food of choice. It stuns its prey using powerful blasts of water from specialized glands in its body. Because of its vicious nature, the people of Kyoshi Island don't swim in the bay where it lives. However, unagis do have an instinctive fear of anything bigger than them.

When the Fire Nation attacked the village on Kyoshi Island and set it on fire, Aang leaped on the unagi and, using the barbels, forced it to spew water, thus putting out the flames.

Trivia

 * "Unagi" is the Japanese word for freshwater eels and consequent sushi.