When Zuko was a child, he and his mother, Ursa, would sit together by the edge of a pond in the Fire Nation Royal Palace and feed the turtle ducks with bits of bread. When Zuko imitated how his sister, Azula, fed the turtle ducks by throwing a loaf of bread at one of the ducklings, the mother duck bit his leg in retaliation. Ursa gently pulled the duck from Zuko's leg and placed it back in the water. The mother and her turtle ducklings subsequently swam away.
Several years later, after he had returned to the Fire Nation, Zuko revisited the same pond to feed the small creatures like before. However, when Azula arrived to speak with him, the turtle ducks swam away hastily, disturbed by her presence.
The turtle duck has the head, neck, and tail of a duck, but the body, shell, and legs of a turtle. Its neck, tail, and most of its head are light brown or beige in color. A dark brown stripe runs from its bill back over its head. Its shell is dark green and its scaly underside is light green or off white.
The turtle duck tends to be calm and gentle, however, if one is provoked, or a mother's baby is endangered, it will attack the offender by pecking them with its sharp beak. The turtle ducks also flee from anything perceived as a threat.
As the name implies, this animal is a cross between a duck and a turtle. Duck is an umbrella term for the common water fowl, which includes geese and swans. Turtles are ancient reptiles that have been around since the Triassic period, and there are two to three hundred species of turtles in existence today. The combination of two aquatic animals makes it a great swimmer.
Eska used "turtle duck" as a pet name for Bolin during their relationship.
↑From older Avatar: The Last Airbender official site, originally on Nick.com (link). No longer updated.