Otter penguin

Otter-penguins are animals used for recreation in the polar Water Tribes. Although a bit clumsy on land, they are very talented and graceful swimmers. They seem to love sliding down the icy shelves and hills of the poles, with or without a human passenger, and they were used as sleds by Katara and Aang. These creatures are native to the South Pole.

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 the Avatar World in many ways resemble the penguins in real life, specifically the emperor penguins and other larger breeds. The primary differences between the otter-penguins and penguins in the real world are the two extra flippers, and the otter-like face. Real life penguins are birds and thus have beaks and feathers.

Behavior
Despite them being rather inept on dry land, the otter-penguins are very capable in the water. Their rubbery skin also makes them good for sliding down ice-slopes. Using the creatures as sleds is a common recreational activity in the Southern Water Tribe, and, to an extent, in the Northern Water Tribe. The animals don't seem to mind, since they enjoy sliding down the ice in this manner, and the added weight allows them to slide faster than normal. The otter-penguins travel in large groups when on land, and most likely when in the water. Although they will flee if chased, they can easily be lured with fish. Otter-penguins are carnivorous, their diet presumably consisting of mainly fish. Otter-penguins have one major natural predator, the polar sea lion.

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 referred to as 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.