He was from a Viking Clan after all. Viking's were honest, loyal, proud, brave, united, and loud, of course. They could swing hard and swig harder. Vikings were Gryffindors and Hufflepuffs and to be sorted into any other house proved you lacked True Vikings Spirit.
Vikings didn't go into Ravenclaw. That was a place for Greeks and scholars and those pompous Englishmen.
Vikings certainly didn't go into Slytherin. A Roman would be more suited for such a place of backward dealings, craftiness and lies.
Hiccup could handle the shock of his classmates (a Horrendous Haddock? In Slytherin? How could he ever expect to take over as chief?) and he could handle his father's disappointment in those short-worded letters. Hiccup was always a bit of a loner and the Slytherin lifestyle catered to that just fine. Everyone in Slytherin minded their own business more or less, and most dealings with each other were for business purposes or for personal benefit. That he could work with. Even kids of other houses were easy to avoid given all the secret passages he'd discovered in his free-time explorations.
No, Hiccup's greatest problem was... his Head of House.
Before Hiccup came along, Alvin the Treacherous had a good, loyal hold on his house. The Slytherin students feared and obeyed him. They maintained a barrier, that aura of seclusion, which separated the Sytherins from the rest of Hogwarts to the point where they were known collectively as Outcasts.
Then Hiccup was sorted into his house and began to slowly undo everything Alvin had spent years, as a student and staff, working for. Hiccup introduces inter-house unity one student at a time to the Slytherin students. First he made friends with that Ravenclaw, Fishlegs. Then he made friends with that Gryffindor, Camicazi.
Now all he needed was to finally worm his way into the heart of that popular Hufflepuff, Astrid Hofferson, and her entire following would be mingling with Slytherins.
No, Alvin the Treacherous certainly did not like Hiccup Horrendous Haddock the third. He didn't like having all his years of careful crafting and planning at Hogwarts being undone. After all, he couldn't take over the school without his house of Outcasts behind him.
So Alvin spends his time trying to get Hiccup expelled and, realizing this, Hiccup turns and tries to get Alvin fired. It's a constant, subtle battle between the two that lasts Hiccup through his years at Hogwarts.
Good thing ol' Headmaster Wrinkly is there to maintain the peace.
In the early years of Hogwarts, before Quiddich became the prime-time of flying, before brooms were standardized and patented by companies, all students were expected to have a rudimentary knowledge of flying and broomcraft. Every student had to craft their own broom as a requirement to graduate. By second year, students would venture as a class into the Forbidden Forest to collect the broom wood. It was up to the individual to choose the core in their own time.
Southerners preferred unicorn-based cores, Easterners preferred anything of hair. Vikings preferred dragon-based cores.
From years 2 to 7, students would take on their personal project of completing their broom. Some crafted brooms early and flew daily. Other never flew at all. There were even those who waited until the months before graduation.
Dragons filled the forest in those days. Nadder spines and Zippleback teeth littered the forest floor. Gronkles left rock-like hunks of skin in tree bark and the occasional Nightmare whisker could be found hanging from a branch after some low flying. Still, the risk of running into, and fighting, these dragons was always a danger.
Naturally, Astrid Hofferson faced off a Nadder, collected a spine, and completed her broom in the early months of her third year.
Hiccup Horrendous Haddock finished his broom at the very end of his fourth year, nearly a year after every other Berkian had completed their brooms. No one knows what dragon Hiccup encountered, or if he even encountered one at all.
All they know is that his broom-core was midnight blue in color, and that Hiccup's broom was the fastest broom Hogwarts had ever seen.