Naga

Naga is a polar bear dog that came into the possession of Avatar Korra as her companion, animal guide, and main mode of transportation. Despite her frightening appearance, she is described as a "sweetheart" and is regarded as Korra's best friend. Undyingly loyal, she is very protective of Korra and has learned how to use her strength to help Korra out of perilous situations.

History
At some point, Naga met Korra and became her animal guide. Korra rides Naga like a horse, even into battle, where the animal proves to be quite agile, able to dodge moving cars at high speed and move quickly enough to keep up with them.

Naga later accompanied Korra when the Avatar escaped to Republic City, where the polar bear dog was seen catching fish from the pond in the city's park. As the guard came running telling Korra not to fish, the Avatar escaped on Naga. She later helped Korra evade arrest by the metalbender cops, but she and Korra were eventually captured and sent to Police headquarters.

While attempting to find Bolin, Naga was quick enough to keep pace with an Equalist convoy consisting of a truck and motorcycles. When Korra and Mako had their chi blocked and found themselves surrounded, Naga bit through her bindings, and she and Pabu scared the Equalists away allowing the two to escape.

Trivia

 * Korra and Naga greatly resemble San from Princess Mononoke and her wolf foster-brothers and foster-mother, the wolf goddess Moro, in both posture and physical traits. The creators admitted to having been inspired by Hayao Miyazaki's movie when creating Avatar: The Last Airbender.
 * Naga was based off the original early concepts of an anthropomorphic polar bear dog traveling with Aang that eventually evolved into Appa.
 * Naga is the Sanskrit and Pāli word for a deity or class of entity or being, taking the form of a very great snake in Hinduism and Buddhism. Naga can also mean 'go out' in Korean (나가), and "dragon" in Indonesian. In Inuit, it means "no".
 * Naga is an anagram of Aang.
 * Naga is the first known polar bear dog to ever be tamed.
 * While both of the known animals that precede Naga in the position of guide to the Avatar were the animals that their people learned their bending art from, Naga is just a regular animal, though this had to be the case since the Water Tribes learned waterbending from the Moon and Ocean spirits.
 * The symbol on Naga's chest is the same symbol that Tonraq wears on his chest.
 * Naga is modeled after the creators' dogs, who are brothers.