Elbow leech

The elbow leeches is a large, parasitic worm with a small mouth on one end that it uses to attach to people's elbows. It live in the murky waters of the foggy swamp. The species' name arose due to its preference for attaching specifically to the elbows of humans and most likely other mammals as well. Once attached, it sucks the host's blood to feed.

History
After a freak tornado sucked Appa and his riders down into the swamp, the gang got separated from the animals. When Sokka emerged from the water, Katara alerted him that an elbow leech had attached itself to his elbow, to which Sokka frantically reacted by asking where the animal was. He pulled it off with a lot of effort and tossed it away.

When Sokka and Katara were separated from the rest of the group in another storm, one of four juvenile delinquents that surrounded them where they landed began touching Sokka's broken arm. For this, Katara disparagingly called him an "oversized elbow leech".

Anatomy
The elbow leech is a large and segmented worm creatures with a small mouth on one end that they use to attach to people's elbows. It has a line of white dots that travel down its side. There is a smaller front sucker and a larger rear sucker.

Connection
The elbow leech is a larger version of the real-world leech. Leeches possess both front and rear suckers, though the front sucker is often very small.