|"It’s a legendary subterranean beast that hunts by scent. Supposedly it can track its quarry across stone, water, dirt, thin air."|
|— Lao Ge about shirshu.|
The shirshu is a large, quadrupedal creature found in the southern Earth Kingdom where it lives underground. Its highly developed sense of smell and ability to paralyze a target make it well suited for both tracking and hunting respectively.
From before the time of Avatar Kyoshi, shirshus were used by earth monarchs to execute their political enemies, but the creature stopped being seen in the wild at least a generation before Kyoshi was born and thus faded into legend.
In 296 BG, a collection of shirshus were procured by the head of the zoology department at Ba Sing Se University, Professor Shaw, during an expedition. One of these shirshus was later given to Hei-Ran, who seemingly wanted to display them in the Fire Nation as part of a goodwill tour between the two nations, though in reality she and Jianzhu wanted to use it to track down Avatar Kyoshi and Rangi, Hei-Ran's daughter. After taking the smell of Rangi, the shirshu found its way to Hujiang in the Taihua Mountains, where it caused great damage and panic before being killed by the local daofei.
During the Hundred Year War, a shirshu was given to a bounty hunter as payment by a rare animal collector, who eventually passed the creature onto his daughter, June. The shirshu, dubbed Nyla, became June's inseparable companion.
Early in 100 AG, Nyla tracked a bounty stowed away on Zuko's ship in one of June's missions. Zuko later hired June to track down Aang using the shirshu. During this mission, Nyla fought Appa and paralyzed him with his tongue before being defeated by Katara, who bent perfume to fill the air around him, the overwhelming smell effectively blinding him. Confused, Nyla blindly lashed out and paralyzed Zuko and June before running off.
Months later, Team Avatar tried to use Nyla to track down Aang when he went missing. After failing to pick up on his smell, Zuko decided to track down Iroh instead. Nyla sniffed his sandal and took them to the walls of Ba Sing Se as a result.
The shirshu is a large, star-nosed mole-like mammal that has no eyes and thus relies on its highly specialized sense of smell. The animal's face has a large, muscular snout that ends in a collection of small, pink, fleshy tendrils surrounding the nostrils, used to detect scents in the environment, and that effectively allows the shirshu to "see" its surroundings. The shirshu can smell a particular scent over hundreds of miles away, making it an ideal tracker. In addition to its strong olfactory abilities, the shirshu's saliva contains paralyzing agents. Along with its large, sharp set of teeth, the shirshu has a whip-like tongue, tipped with large, triangular papillae, which the shirshu uses to disable its prey. Depending on the target's size and how hard it is hit, the shirshu's tongue can either stun or paralyze prey for a period of time, ranging from a few minutes up to several hours.
Tamed shirshus are mainly used for their superior tracking ability; they can detect a particular scent anywhere in the world, paralyze their prey from afar without doing permanent damage, are extremely fast and agile, and can travel over great distances without tiring significantly. Shirshu saliva contains powerful toxins, making it highly valuable; it is used in the production of paralyzing darts.
Despite its size and weight, the shirshu has outstanding agility and can hold its own in close combat situations. Using its powerful claws, it can easily protect itself from most threats, though in the majority of cases, close combat is unnecessary: the shirshu's tongue can reach up to several meters and incapacitate opponents before they have a chance to get near. Its critical weakness is its highly sensitive sense of smell, which can be overwhelmed by strong scents, effectively "blinding" it and, in some cases, inducing a frenzied panic.
Nyla's semi-domesticated temperament was determined, obedient, and he was socially affectionate, trying to lick his handler, June, after she fed and cuddled him, and he allowed Appa to lick his face before settling down by the sky bison after they initially growled at each other. When commanded, he was also ferocious, responding instantly to June's whip, and he showed embarrassment and shame when he could not pick up Aang's scent from one of his belongings.
The shirshu is a combination of a giant anteater, a wolf, and a star-nosed mole, which engenders rumors that it has excellent digging ability. Instead of digging with its nose, the shirshu uses it much like how a bloodhound uses its nose in the real world. Additionally, some species of moles, such as the European mole, are venomous, matching the shirshu's paralyzing tongue.
- "For the traitor, let them be hounded by shirshu until they drop where they stand, far from their homes and the bones of their ancestors" is a saying about shirshus.