Both the book series version of Hiccup Horrendous Haddock III, and the franchise version of Hiccup Horrendous Haddock III wore a Viking helmet. In How to Be a Pirate, Hiccup's Viking helmet had part of one horn sliced off while swordfighting.
Hiccup usually wears a helmet in the books. Toothless sometimes sits on top of it or hides under it.
Hiccup is given the Slavemark by Bearcub's gradmother as a way to make him keep his promise that he would free her annd the rest of the wanderer slaves. Hiccup uses his helmet to hide the Slavemark.
Hiccup loses his helmet when Snotlout thows a rock at it thus knocking it off his head, revealing his Slavemark. Hiccup leaves the helmet behind during the dragon attack at Flashburn's School of Swordfighting. Fishlegs puts on the helmet after Hiccup escapes as a way to remind himself and Camicazi of Hiccup's bravery. What happened to the helmet after that is unknown as when Fishlegs is seen again in Book 10 he lacks the helmet.
During the between How to Steal a Dragon's Sword and How to Seize a Dragon's Jewel, Hiccup stole a new helmet to keep undercover. However Hiccup wasn't fond of the helmet as it was too big and uncomfortable.
Hiccup lost this helmet as well when Snotlout was pretending to be him so he could escape. Alvin the Treacherous and Excellinor the Witch obtain the helmet, and show it to the Dragonmarkers as proof that Hiccup is dead, though in reality Hiccup survived, because Snotlout was pretending to be Hiccup so he could escape.