Well, Hiccup is very resourceful. He could use the environment around him to his advantage. If he is locked in a place, he might find a weak spot in the structure, like a loose board or a hole in the wall. And he could call for Toothless secretly. Toothless, being highly intelligent, could find Hiccup and rescue him. Also, Hiccup could befriend some of Viggo's minions and convince them to help him escape.
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.
There may be some extremely creative and boundary - pushing fanfiction out there with this premise, but it's not something that is typically found in the more well - known or widely - accepted "How to Train Your Dragon" fanfiction. Usually, fanfictions in this fandom focus on things like their adventures, friendship, and the development of their bond in a more traditional sense. So, while it's possible, it's not a common plot line.
Toothless is definitely one of the most popular. Since he's Hiccup's closest dragon companion in the original story, in fanfics where Hiccup is a shapeshifter, Toothless can have a whole new dynamic with him. He might be the one to help Hiccup understand his new abilities, or be protective of him when others are suspicious. Astrid is also popular. She could be the one who discovers Hiccup's secret and has to decide whether to keep it or tell the others. Her relationship with Hiccup, which is already complex in the original, can take on new twists and turns in these fanfics.
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.
In the fanfiction, perhaps there's a moment when Hiccup gets injured while protecting the village. Astrid stays by his side day and night, taking care of him. She reads to him, makes him his favorite food, and in those quiet days, they realize how much they mean to each other. Their love deepens as they share these tender moments.
It develops gradually. At first, they might be just friends who respect each other's skills. But as they go on more adventures together, they start to notice little things about each other. For example, Hiccup notices how Astrid is always the first to volunteer for dangerous tasks because of her bravery. And Astrid sees how Hiccup's intelligence always gets them out of tough situations.
Astrid is an obvious choice. She already has a close relationship with Hiccup in the original story. Then there could be Ruffnut and Tuffnut's sister, who might see Hiccup in a new light and be attracted to his growing confidence. Also, some new characters from unexplored Viking tribes could be part of it. Maybe a healer from a far - off land who is intrigued by Hiccup's connection with dragons.