Otter penguin

The Otter-Penguins appear in "The Boy in the Iceberg" when Aang asks Katara if she would like to go penguin-sledding with him.

Anatomy
Otter-Penguins are aquatic, flightless mammal-like birds found mainly in the South Pole. They have four flippers and are approximately as tall as a teenager. Their faces are somewhat otter-like, with small black noses and whiskers. They have short necks that are almost indistinguishable from their chests. Their feet also seem to be webbed. Their coloration is counter shaded, meaning their backs, tail, flippers and head are black and their underbellies are white.

Connection
The Otter-Penguins in Avatar resemble in many ways the penguins in real life, specifically Emperor or other large breeds of penguin. The primary differences between the Otter-Penguins on the show and those in the real world are the two extra flippers and the otter-like face. Real life penguins are birds and so have beaks and feathers.

Behavior
Although rather clumsy on land, Otter-Penguins are very talented swimmers. Their rubbery skin also makes them good for sliding down ice-slopes. Using the creatures as sleds has been a popular Southern Water Tribe child's game. The Otter-Penguins stay in big groups when on land and most likely when in the water. Otter-Penguins have one major predator, the Tiger Seal.

Trivia

 * The Otter-Penguin is often called a penguin, which is false.
 * In "The Boy in The Iceberg" with "Avatar Extras" bonus commentary, Otter-penguins are called Penguin-Otters.
 * In "The Boy in The Iceberg" with "Avatar Extras" bonus commentary, it is revealed that the whiskers on Otter-Penguins are inspired by the mustache style called the Fu Manchu.