Disclaimer: I do not own "How to Train Your Dragon."
Separate, they were crippled.
Hiccup had lost his leg in the battle with the Red Death, and although it was much better alternative than falling to his death, walking would never be the same. The prosthetic had helped to at least make him move around on his own, but no mechanical creation forged by man could ever replace the organic original; the foot, so often taken for granted, plays an essential role in locomotion. There are so many unconscious adjustments made by the muscles and nerves in aiding with balance and stability, that no piece of metal, however cleverly crafted, could ever truly mimic it. Hiccup could walk, but his limp was very noticeable, and although that limp became less pronounced as he relearned how to put one foot in front of the other, it would never be the same.
Toothless had lost half of his tailfin the night Hiccup brought him down, and although losing that fin was a better alternative than being killed by a Viking, flying would never be the same. The tailfin that was so often taken for granted became a missed friend in the air. While it was his large wings provided the primary lift and locomotion for flight, it was the secondary wings and tailfins that gave the Night Furies the speed and agility that had made them so infamous and terrifying to the Vikings and even other dragons. Without that caudal rudder to make those subtle changes in flight, the best Toothless could manage were brief gliding sessions. He could fly again, thanks to Hiccup's prosthetic, but he could no longer steer by himself, and taking to the air would never be the same.
And they wouldn't have it any other way.
Separate, they were crippled. Together, they were wondrous.
Once Hiccup sat in the saddle, and Toothless took to the air, they became one organism. The countless number of hours they had spent training together culminated in an art that was the envy of even other Night Furies. In the beginning, it was Toothless that did the majority of flapping, while Hiccup did his best to catch up in the steering. Now, once Hiccup hooked his prosthetic foot into the stirrup that controlled Toothless' prosthetic fin, they flew in perfect harmony, as if they were of one mind. No other such bond existed, and it was improbable such a deep and profound friendship between Viking and dragon would ever occur again.
Separate, they were crippled. Together, they were more than wondrous; they were whole.