The shirshu is a large, star-nosed mole-like mammal, that has no eyes, but its inability to see is made up for by 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, 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 neurotoxins. 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 take down 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 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 neurotoxins, 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 as 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.