Naga

Naga is a Polar Bear Dog that somehow came into the possession of Avatar Korra as her animal guide.

History
Most of Naga's history is unknown, however, at some point, she met Korra and became her animal guide. Korra rides Naga like a horse, even into battle. This creature is a Polar Bear Dog.

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 this anime when creating Avatar: The Last Airbender.
 * Naga is based off the original concept designs of the previous Avatar's animal guide, Appa.