Eel hound

Eel Hounds are used as riding animals when Sokka, Toph and Suki rode one to intercept the Fire Nation air fleet. They look slightly similar to a Greyhound, and can run at similar speeds.

The Eel Hound serves as a deus ex machina in the final episodes to explain Sokka and his friends' rapid movement from Ba Sing Se to the Fire Nation Airship Base.

Anatomy
An Eel Hound is a large, amphibious lizard that exhibits the ability to swim and run very quickly. They are at least 8 feet tall and roughly 25-30 feet long, with dark green scales except along its belly, from its jaw to its tail. Eel Hounds are green in color with a lighter shade on their underbellies, and a darker shade of green on top of their bodies.

Behavior
The Eel Hound is very hard to tame, although nothing is known about them outside their domestic environment. They were mentioned by Piandao to be the fastest thing to ride or swim: "Nothing runs faster over land or swims quicker than a giant Eel Hound," most likely referring to animals used as means of transportation, as there are animals which run and swim faster.