Naga

Naga was a polar bear dog that somehow came into the possession of Avatar Korra as her companion and animal guide.

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

Trivia

 * The image of Korra's animal guide was unveiled at San Diego Comic Con 2011.
 * Korra and her spirit animal greatly resemble San from Princess Mononoke and her wolf in both posture and physical traits. The creators admitted to having been inspired by that movie when creating Avatar: The Last Airbender.
 * Naga was based off the original early concepts of an anthropomorphic polar bear dog traveling with Aang.
 * 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."