There are so many fantastical abilities in the world of Avatar: The Last Airbender that it's easy to take some of the less showy abilities for granted. When benders are manipulating the blood in their opponent's veins or throwing out lightning from their fingertips, it can be hard to appreciate just how difficult something like flying is. But with its masterful storytelling and world-building, Avatar managed to convey an amazing history behind the art of flight most fans may not even pick up on. You just have to look for it.
At first glance, the ability of flight may not appear to be all that rare in Avatar. After all, flying bison like Appa and Aang's glider are both front and center in the beginning of the original series, and there's more than enough of both when the Air Nation restarts in The Legend of Korra. But once you start to consider true flight, unassisted by outside help like animals or gear, the power of flight becomes a lot rarer.
What's interesting when reviewing both series for examples of flight is that most instances either come from the Avatar State or a firebender rather than one of the usual Air Nomads you would suspect. Much of the time that Aang flies without the aid of his glider, he does so while tapping into generations of experience and power through the Avatar State, rising up on a column of air or surrounding himself in a bubble of the elements. Tapping into external forces of power proves to be a necessity for the firebenders who attempt flight as well.
The closest any non-Avatar comes to true unassisted flight in the original series all happen during Sozin's Comet, when firebenders' powers are magnified immensely. Azula, previously only capable of boosting herself through the air or skating across the ground with fire, becomes so empowered by Sozin's Comet that she can support her full weight in midair. Jeong Jeong and Ozai also spend extended periods of time without touching the ground when they're powered up by the comet.
The trend continues in The Legend of Korra, where firebenders like Iroh II, Korra and Mako are all capable of boosting themselves with jets of flame but are never capable of taking off from the ground and flying through the air. This suggests that the ability to fly is incredibly hard, requiring either the Avatar State or Sozin's Comet to fully achieve. But that's when Zaheer swoops into the story.
Formerly a nonbender, Zaheer was granted the gift of airbending after Harmonic Convergence. But even prior to gaining the ability himself, Zaheer was a scholar of Air Nation philosophy and dedicated himself fully to the Guru Laghima. Existing at a time thousands of years before the original series, and even before the formation of the Four Nations, Laghima allegedly mastered the art of self-sustained flight and did not touch the ground for the last 40 years of his life.
Though Laghima's philosophy and poetry survived him, few believed that he literally discovered flight. That is until Zaheer rediscovered the ability, unlocking the ability to "let go your earthly tether" and "enter the void" after the death of his love, P'Li. Zaheer proved to be the only bender capable of perfect unassisted flight in the modern era, the only completely unique bending sub-skill in Avatar. And yet, there was another, simpler form of flight that predated him.
It was actually in Season 2 of The Legend of Korra when viewers first saw a form of flight possibly superior to Zaheer's. In the time of the first Avatar, Avatar Wan, who lived 10,000 years ago, the first Air Nomads gained their ability to manipulate air currents from a lion turtle that floated in the sky. Unlike any Air Nomad seen since, those originators stood on puffy clouds they conjured beneath themselves. Wan even gained the ability and showed an impressive degree of speed on the clouds -- all without outside assistance, and all without spending years of meditation and practice.
And yet the ability has never been seen against since the time of Wan. Despite the Avatar State granting a connection to all previous Avatars, it seems that none have ever replicated Wan's ability to ride along on self-conjured clouds. So much of the history within the world of Avatar became lost over the ages, and the wars and destruction in the time of Kyoshi and Aang have not helped. But the creators behind the world of Avatar were so subtle about telling the story that the history of the ability flew right over most viewers' heads. All this time, they just had to look up to appreciate it.