A/N: I took many liberties with this prompt + interpreted it as 'a potentially dangerous + fun thing to ride', which turned into dragons. And then I took many liberties with canonical dragons.
motorcycle (n) - a two-wheeled vehicle that is powered by a motor and has no pedals
The message comes from Iroh on a clear autumn afternoon—Fire Lord Zuko's presence is required in the Sun Temple as soon as he can set his affairs in order.
Zuko reads and re-reads the letter until he feels a headache start to build behind his eyes, but he can't find any hidden message, any clue as to why he has been called to visit the civilization he hasn't heard from since he was sixteen.
Katara looks up from her habitual spot where she lounges in one of the meeting chairs of his office—when she's not peering over his shoulder, reading his papers and offering suggestions on policy—and says that perhaps they finally decided he should offer reparations for the damage he and Aang did when they visited years ago. Zuko hadn't offered reparations to the minor tribes like the Sun Warriors or the swampbenders or the sandbenders, after all.
Zuko frowns and she smiles her most practiced, innocent smile at him.
(Sokka's recent visit has done little for Zuko's wife's sense of humor.)
When Zuko arrives with as little fuss as possible, he finds the Temple in the midst of an uproar of celebration: the dragon egg that the tribe has guarded so carefully for years, that was hidden away since Roku and Sozin's time—the one that Zuko so thoughtlessly swiped from its resting place in the sanctuary—has hatched.
Dragons are loyal creatures, Zuko discovers, and because he unwittingly triggered this one's birth, it has what he considers an overdeveloped attachment to him. (Later, Katara will bend water at him before laughing and kissing him and telling him, Of course it likes you. You're a very likable person. Zuko thinks the Fire Nation heat must have gone to her head, because he is many things—awkward, overwhelmed, partially redeemed—but likable has never been one of them.)
When he returns to the capital, he works with Iroh and Katara to decide the best way to tell his people the news. Because the dragons have been rumored to be extinct for so long, because Iroh's already-damaged reputation is marginally at risk for his lie to save the dragons' lives, because it is still considered a mark of honor, culturally, to kill a dragon, their reintroduction has to be handled delicately.
In the end, they recruit the help of the Fire Sages, create a holiday to honor the dragons and the history of firebending, and Zuko sends part of his royal guard to assist the Warriors in guarding the creature during the first few years of its life.
There are only fifteen poaching attempts before the craze calms down, and all dragons and defending soldiers are unharmed, so Zuko considers the move a success.
When other dragons have joined the first one and the dragons' return has been firmly entrenched in the people's minds, the Sun Warriors send "his" hatchling—now standing as high as Zuko's shoulders—back to the capital with him after a visit.
On another sunny autumn day, Zuko sits at a low tea-table with Katara and Iroh and drinks another one of Iroh's experimental blends as they watch the dragon explore its new home in the capital.
Katara comments with a smile that she hopes having the dragon live in the royal compound causes less trouble than when Aang tried to help the zookeeper with his animals in Ba Sing Se—a story Iroh has heard rumors of in his tea shop in the city, but the details of which Katara can provide with much more accuracy.
Then she looks thoughtfully at the dragon and asks, "So, when do we get to ride it?"
Zuko chokes on his tea and splutters and is thankful that their children aren't around because they haven't thought to pester him about that yet where dragons are concerned, but before he can think of an answer, Iroh smiles widely and begins to discuss the history of riding dragons and of their use in both warfare and surveillance, and they sit for a long time listening to tales of Roku's dragon, who was his companion as the Avatar just like Appa is Aang's.
The answer to Katara's question is four months later, when the dragon is old enough to be trained and broken, and after Zuko has grasped the idea that he and his wife can more or less safely ride a dragon without being smitten by Agni for sacrilege, it becomes one of their favorite pastimes to sneak away when they can and visit the dragon's compound.
At Katara and Iroh's insistence, Zuko rides the dragon at the next Dragon Festival and spends the next several weeks teaching his children how to ride safely, as a result.
When he joins Katara at the side of one of their children's lessons to watch as they receive additional instruction from one of the dragon's handlers, she wraps one arm around him and leans up to whisper, "See? I told you riding the dragon was a good idea."
And, as with most disagreements with Katara, Zuko finds that, in the end, he has to agree.