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 could be a young girl from a far - off land who has a special bond with a rare type of dragon. She and Hiccup could join forces. She might have unique skills like being able to communicate with dragons without words. And also, an old hermit who has been living alone with his dragon for years. He has a lot of wisdom about the ancient ways of dragon riding and could help Hiccup on his travels.
There could be Toothless. Even though Hiccup is now a dragon, his connection with Toothless from his previous human life might still be there. Toothless could play a crucial role in helping Hiccup adapt to his new form.
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.