With every episode of Avatar: The Last Airbender, it became even more evident that the creators of the show loved animals just as much as they did martial arts. Over the course of Avatar, as well as its direct sequel, The Legend of Korra, fans were introduced to a myriad of animals, many of which being integral companions that weren't just cute and cuddly, but served as pivotal supports to their owners in their quests to end wars or stop leaders with chaotic agendas.
And what makes them all extra noble is that they did their duties without asking for a single belly rub, although many were offered.
Bosco
Besides being a rare specimen of animal in the Avatar world, there's not really anything special about Bosco. After being pampered by the Earth King for so many years, he grew to be pretty stocky, even for a bear, but he shares a lot with his sheltered owner.
After losing their status when the Fire Nation took control over Ba Sing Se, the two traveled the world that they had been denied from seeing all their lives. Bosco stayed by his owner's side, and despite being forced to give up the trappings of his royal lifestyle, didn't complain one bit.
Nyla
Nyla was the shirshu that belonged to the bounty hunter June. As seen in "Bato of the Water Tribe" and "Sozin's Comet", without her keen sense of smell and paralyzing tongue, June wouldn't have been able to maintain her reputation as one of the Earth Kingdom's best bounty hunters.
Though she accidentally struck June after her sense of smell was disoriented by an overwhelming mass of perfume, she otherwise obeyed June's commands without question. While it's clear that she has a bit more training to do to prevent that from happening again, the bond between Nyla and her owner is one that's just as frightening as it is unbreakable.
Flopsie
When Flopsie was first introduced, fans were surprised that an old man like Bumi had been able to tame such a gargantuan beast. Despite his outward appearance, Flopsie is near-harmless, and it's clear that he cares a lot about Bumi.
When Aang searched a captured Omashu, it was Flopsie that led him to the rebels, doing his part in aiding Team Avatar's efforts to reclaim the city for the Earth Kingdom.
Pabu
Pabu's cuteness isn't the only thing he's got going for him. He was essentially the inspiration behind Korra, Mako, and Bolin's Pro-bending name, the "Fire Ferrets". When Bolin and Mako were just kids, Bolin bought Pabu from a pet shop, saving him from his fate as another animal's dinner. Since then, Pabu has been one of the most loyal animal companions in the Avatar universe, staying by Bolin's side throughout some of his darkest moments.
When the team needed money to enter the Pro-bending championship tournament, Pabu helped Bolin by performing tricks on the street to the best of his ability. When Team Avatar found themselves tied up by Amon's men, it was Pabu who chewed through the ropes, and after Korra had been kidnapped by the Red Lotus in Zaofu, Pabu's keen senses woke him up, enabling him to warn Bolin just in time. Despite being so small, he's made a huge impact on Team Avatar's success, all the while maintaining that cute face of his.
Druk
In Avatar: The Last Airbender, fans had been led to believe that dragons had long since been hunted to extinction by the Fire Nation, but Book 3 introduced two that actually managed to survive the genocide of their race. Druk is a descendant of those two dragons, and served as Zuko's animal companion during his later days as a Firelord, and even well into his retirement.
There haven't been many emotional scenes between the two, although it is notable that Druk attempted to aid Zuko during the battle against P'Li and her teammates after she'd been freed.
Mula
Like many Avatars that came after him, Avatar Wan had his own animal companion in the form of Mula, a cat deer.
The two were almost like kindred spirits; shortly after Wan was forced to live in the Spirit Wilds, he saved Mula from a trap set by other humans, where he not only proved that he was different from them, but also proved that he was willing to coexist with the creatures of the Spirit Wilds. The two traveled the world together, and Mula watched as his owner became the first master of all four elements.
Fang
Roku raised Fang from the time he hatched from an egg, creating an unbreakable bond between the two. Most notably, Fang tried his best to help Roku quell the volcanic eruption on the very island they called home, but when it looked like Roku would perish, Fang decided to spend his final moments with his master.
It was reassuring to know that the two were together even in the Spirit World, proving that friendships can, in fact, transcend the living world itself.
Momo
When Aang nearly fell off of the Fire Nation's Drill, Momo helped pick him back up, enabling him to deal the final blow before Azula could recuperate. It's no surprise he and Appa became such good friends; anyone would find themselves being drawn to Momo in the same way.
Naga
Naga is a polar bear dog that had a bond so close with Korra that she often spoke to her the way Aang spoke to Appa. Not only was she the only one by Korra's side when she traveled to the unfamiliar territory of Republic City, but she aided her in many important fights during her time in the city. At one point, Naga smacked Amon's Lieutenant right out of the air, sparing the others the trouble of dealing with him in a normal fight.
In that same season, she freed Bolin, Asami, and Iroh after they'd been captured by Hiroshi Sato's men, using her raw strength to break down cell bars that no one else could've. After Korra's disappearance for over half a year, it's certain that no one missed her more than Naga, who she'd been there for ever since she was just a pup.
Appa
Aang and Appa's friendship is one that is just as symbolic as it is pure. The two met long before the days of the Hundred Year War, but after waking up a century later, they instantly became two relics from an extinguished culture, with Aang being the last Air Nomad in existence and Appa being the last flying bison. The fact that Appa missed Aang just as much after the two were separated showed just how deeply connected they were.
It was heartbreaking in The Legend of Korra to see an adult Aang without his flying bison, since it was clear that Appa passed away, but thankfully, with the discovery of more flying bison in the world, fans can rest assured with the realization that Appa's legacy can live on in a new generation of flying mammals that defied just about every law of physics, just like their owners.