The shirshu is a large, quadrupedal creature found in the southern Earth Kingdom. Its highly developed sense of smell and ability to paralyze a target make it well suited for both tracking and hunting respectively.
The shirshu is a large, star-nosed mole-like mammal that has no eyes and, therefore, relies on its highly specialized sense of smell instead. The animal's face has a large, muscular snout that ends in a collection of small, pink, fleshy tendrils surrounding the nostrils, which are used to detect scents in the environment, and 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 smelling 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 German Shepherd, 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.
Its name comes from a combination of the Chinese names for "anteater" (食蚁兽) and "mole" (which is 鼹鼠).
↑ 1.01.1From older Avatar: The Last Airbender official site, originally on Nick.com (link). No longer updated.