Toothless is the most likely one. Their bond is so strong that Toothless would sense Hiccup's distress and come to his rescue. He could fly into Drago's hideout and cause chaos to free Hiccup.
Hiccup's friends would be extremely worried. Astrid, being Hiccup's closest friend and love interest, would probably be the first to take action. She would gather the other riders and start planning a rescue mission right away. Snotlout might initially be a bit hesitant, but the thought of losing Hiccup would push him to join in. Fishlegs would use his knowledge of dragons to figure out where Drago might be hiding Hiccup.
Hiccup could try to resist by using his intelligence. He might find loopholes in Dagur's plans or demands. For example, if there are certain cultural or traditional rules that Dagur is using to force the marriage, Hiccup could find ways to show that those rules are being misinterpreted or misapplied.
In 'hiccup the dragon fanfiction', one aspect could be the different dragon species and their characteristics. Hiccup might interact with various dragons, not just Toothless. It could show how he learns to communicate and cooperate with different types of dragons. Also, it might include some emotional parts, such as the bond between Hiccup and his dragon when they are in difficult situations, like being separated or facing a life - threatening danger.
His relationships with others can also be a big part of his character development. If he meets other captives or his kidnappers, how he interacts with them, whether he tries to understand their motives or just fights back blindly, can show his growth. Maybe he starts off being angry and vengeful but then realizes that there's more to the situation and becomes more empathetic.
Astrid could be one of the main characters. She might follow Hiccup to the marines to keep an eye on him or to have her own new adventures. Another character could be a tough marine sergeant who initially doubts Hiccup but later comes to respect him. There could also be a marine scientist who is interested in Hiccup's knowledge of dragons.
Yes, he might. Hiccup is a character from 'How to Train Your Dragon' series. If there is an interesting HTTYD 2 fanfiction out there, it's possible that someone could imagine Hiccup reading or being involved in it in some way in the context of fan - created stories.
The setting is important too. It could be the familiar Berk or some new, uncharted lands. A different setting can bring new challenges and opportunities for Hiccup. For instance, if it's in a land filled with ice and snow, Hiccup has to deal with the harsh environment as well as any threats that might lurk there. Also, the other characters from the original story, like Astrid or Stoick, might play different roles in the fanfic, either supporting Hiccup or creating conflicts for him.
I'm not entirely sure specifically what 'hiccup black fanfiction' is about without reading it. But generally, in fanfiction, when something is 'black', it could imply a darker take on the source material. So for Hiccup, it might be about him facing moral dilemmas, being involved in a darker plotline like in a world full of chaos or corruption, and how he navigates through it in this fan - created version.