Who's helmet does the dragon that requires a prosthesis bring back?
Right before Berk's traditional winter holiday of Snoggletog, all the dragons of Berk unexpectedly depart, leaving everyone distraught - except for Toothless, who cannot fly by himself. Out of compassion, Hiccup builds him a new automatic prosthesis allowing him independent flight, thus gifting him his freedom; he then flies off too.
Three days later, Meatlug, Fishlegs's dragon whom he had secretly kept chained, escapes, inadvertently taking Hiccup with him. Meatlug flies to an island with hot springs, where all the dragons (except Toothless, whom is nowhere to be found) have been hatching their eggs. Meanwhile, on Berk, Astrid and the other youth discover dragon eggs in Meatlug's nest, which they scatter around Berk in hopes of lifting the villagers' spirits; this plan backfires because dragon eggs hatch explosively (normally underwater), devastating Berk.
On the dragons' island, Hiccup meets Stormfly (Astrid's dragon) and Hookfang (Snotlout's dragon) and their newly-hatched babies; in asking for a ride home, he inadvertently triggers the dragons' return migration. He quickly decides to use a nearby wrecked ship to carry the baby dragons who cannot yet fly all the way back to Berk. The Berkians are overjoyed at their dragons' return and the new babies; yet, Hiccup is still distraught at Toothless's absence. During the ensuing Snoggletog celebration, Toothless returns with Hiccup's lost helmet, which he had dropped into the sea earlier; the two enjoy a heartfelt reunion.
The next day, Toothless destroys his new tail, begging Hiccup to put the old tailfin on him and fly with him by controlling his fin manually as opposed to merely on him; in doing so, he gives Hiccup a "better gift" - his friendship and companionship.
Hiccup