Eel hound

Eel hounds are creatures used for fast transportation over water and land.

History
Piandao provided Sokka, Suki, and Toph with one of these creatures to transport them to Ozai's airship base in order to stop his airship fleet, while Katara and Zuko journeyed to the Fire Nation Capital on Appa's back. The eel hound quickly crossed the Earth Kingdom and swam toward the base, allowing its riders to board the departing fleet at the last moment.

Anatomy
An eel hound is a large, amphibious lizard that exhibits the ability to swim and run very quickly. They are at least eight feet tall and roughly twenty-five to thirty 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.

Connection
The eel hound is a cross between a greyhound and an eel. It has the long, powerful legs, deep chest, flexible spine, and slim build of the greyhound combined with the elongated neck of the eel. They look slightly similar to a giant-sized greyhound, and can run at similar speeds.

Behavior
The eel hound is a wonderful animal once it is trained; they can be pretty wild otherwise. But training an eel hound is very difficult, so most people won't even bother. This is the reason why they aren't as commonly used these days. 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.

Trivia

 * The eel hound served as a deus ex machina in the final episodes to facilitate Sokka's, Toph's, and Suki's rapid movement from Ba Sing Se to the Fire Nation airship base.