Nyla was a shirshu who served as the mount and constant companion of the bounty hunter June. June acquired Nyla from her father when he received him as payment for a bounty he had collected while working for a rare animal collector. Nyla and June had been together for over seventeen years. Like other shirshus, Nyla's tongue contained toxins that could temporarily paralyze humans, though the effect was less potent on large animals such as Appa.
In one of June's missions, Nyla tracked a bounty stowed away on Zuko's ship. He bit into the metal deck, tearing a large piece out to reveal the stowaway. Zuko hired June to track down the Avatar using the shirshu. When Zuko tracked and attacked Aang, Nyla fought Appa and paralyzed him with his tongue, although it took several hits to bring the flying bison down. Katara defeated Nyla by confusing him with perfume which she bent out of pots in the abbey. Nyla 'blindly' lashed out and paralyzed Zuko and June successively before running wildly away.
Months later, when Aang went missing, Zuko and Team Avatar traveled to the Earth Kingdom, where they found June in a tavern. Outside, Nyla and Appa seemed to have befriended each other. Katara handed Aang's staff to June so the shirshu could sniff it. He turned in circles before covering his nose, unable to find Aang. Zuko decided to track down Iroh instead, and he handed June his uncle's sandal. Nyla sniffed it and led them to the walls of Ba Sing Se, where he and June departed from them.
After the Hundred Year War, June and Nyla were hired again by Zuko, now Fire Lord, to track down Ursa, his long-lost mother. However, the search turned up no results.