Eel hound

The eel hound is a creature used for efficient transportation over water and land.

History
Shortly before the end of the Hundred Year War, Piandao provided Sokka, Suki, and Toph with one of these creatures to transport them to Ozai's airship base in order to stop the Fire Lord's airship fleet. 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
The eel hound is a large, amphibious lizard that exhibits the ability to swim and run very quickly. It is at least eight feet tall and roughly twenty-five to thirty feet long, with dark green scales (except along its stomach) from its jaw to its tail. The eel hound is green in color with a lighter shade on its underbelly, and a darker shade of green on top of its body.

Behavior
The eel hound is a very wild creature. As a result, training an eel hound is very difficult, so most people will never even bother. For this reason, it is not as commonly used these days. The eel hound was mentioned by Piandao to be the fastest thing to ride or swim.

Tamed behavior
If fully and properly tamed, eel hounds can be used as transportation and perform tasks such as swimming over large bodies of water or running on land at great speeds. The eel hound is a tame, calm animal once it is trained.

Connection
The eel hound is a cross between a greyhound and an eel. With its long, powerful legs, deep chest, flexible spine, and slim build of the greyhound combined with the elongated neck and tail of the eel, it slightly resembles a gigantic greyhound and can run at similar speeds.

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.
 * After the Hundred Year War, a pro-bending team from Ember Island named themselves the Eel Hounds.