Speaking to Newsweek for the 10th anniversary of “Sozin’s Comet,” co-creator Bryan Konietzko explained that a lot of the story, the characters, and the world were created as they were pitching the show to Nickelodeon. “We expanded upon many of those ideas in the subsequent months as we created the series bible and then the test pilot.”
Part of that test pilot, which you can watch online, is the lion turtle itself. It appears in an earlier version of the opening title sequence right behind Aang, in massive statue form. “It wasn’t just part of the production design — we were planning its role in the finale even that far back,” Konietzko added. Indeed, though we don’t see the actual lion turtle until the finale, we see images of it throughout the series, like on a white lotus tile, or mentioned by Wan Shi Tong in The Library episode of season 2, or in several statues in the house of Piandao, Sokka’s master.
The lion turtle’s appearance and power may have been a bit sudden, sure, but it is clearly a part of the world long before we meet it. More importantly, it does support the idea we see over and over in both “Avatar” shows, that the spirit world is simply way more vast, ancient, and powerful than we know.