Okay I'm doing this because I wondered this in the movie as well. How did Hiccup lose his leg exactly? Well here it is short and sweet!
(Based off the movie series since I've only seen the movie)
The evil dragon king (as I like to call him) roared as he dove headfirst towards the ground, his tail flailing madly in the air "get us out of here Toothless!" The red haired Viking begged his dragon who nodded and soared away. Or at least tried, Hiccup looked behind his shoulder to see the makeshift tail he had invented was now a burned mess "oh no. Come on Toothless we can make it!"
The nightfury roared as he tried to turn away only to thrash against the tail of the much larger dragon. Hiccup lost his grip on the reins of his dragon and fell. The dragon king's madly flapping tail struck him in the head, knocking him unconscious.
Rushing to his friends aid Toothless dived head first towards the ground where a large explosion shook the earth when the dragon king had finally crashed. With his tail in flames and his flying crooked toothless finally managed to get some distance and was finally close enough to him.
Unfortunately the ground neared and Toothless couldn't reach out far enough past his snout to grab his friend. The dragon wanted to cry as he realized the only thing close enough to reach him was his mouth and that if he retracted his teeth Hiccup would slip away.
Slowly letting his teeth show Toothless violently prayed to the gods of Berk that if Hiccup found out he would somehow forgive him.
The nightfury sunk it's large fangs into Hiccups leg and yanked him towards his chest and wrapped his black wings around his tiny form before crashing hard against the ground.
Toothless awoke first and blinked slowly "Hiccup! Hiccup!"
The nightfury recognized the voice as Hiccups father Stoic. The dragon made a pitiful moan before closing his eyes. Begging to be slipped back into unconsciousness.
The Viking soon saw his son's dragon and ran towards him "oh no." He said when he saw the dragon was alone "he. He didn't make it."
The dragon purred sadly not wanting the father to see his son's foot in it's mangled state, and secretly not wanting to see it either. "You did well though dragon. I take everything back about you creatures. You did your best to save my son. Oh Hiccup."
Astrid pushed her way through the crowd "move it! Out of my way!" She prayed that Hiccup would be standing there grinning goofily at her with Toothless beside him with those big green eyes, but what she saw was the beaten and weary nightfury and a near to tears Stoic. "No, oh please god no!"
Toothless looked to the crowd to see the many hurt faces. He put his head down sadly when Gobber came to lead Stoic away.
Slowly, not wanting to hurt his friend anymore, Toothless lifted his raven wings and revealed his injured, but alive, master.
Stoic quickly but carefully gathered his son into his arms "you brought im' back alive."
Gobber looked over Stoic's shoulder and winced "well most of im'"
Toothless glanced down and let out a terrified screech when he saw Hiccup's entire foot missing how could I? I didn't mean to rip his leg off! Oh dear god what have I done? WHAT HAVE I DONE?!
Seeing the fear and dread in the nightfury's eyes Stoic laid a hand on his snout "ya could have saved yourself and let im' die Toothless, but ya didn't. You risked your life to bring back me son, and I'm sure ma boy will forgive yee. Now ya both are the same."
Toothless looked at his ruined tail and purred lightly "ya both are handicapped now. You can't fly without im' and he can barely walk without ya to guide him."
The dragon let a small crooked smile grace his features.
"my boy needs ya Toothless. So don't go beatin yourself up jus because you made you hurt him. It was for a good reason." The dragon purred again "but don't think I'm not mad. I am, so I'm not letting you near im' until we get back and this leg is taken care of. Got it?"
The nightfury nodded furiously, "good. Now kids if you would be so kind. Would you show us adults how to fly?"
Yay it's done!
I really like how this came out I think it's great.
Disclaimer: I don't own How to train your dragon and I probably never will.
Thank you for reading fans! I may make another one, like a sequel or something I don't know you guys decide. I'm going to bed. Night night!