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.
In this kind of fanfiction, perhaps Hiccup would be in great danger. Drago might use Hiccup as a hostage to get something he wants, like controlling the dragons that are allied with Hiccup's people. He could also try to force Hiccup to reveal the secrets of his unique bond with Toothless.
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.
One possible plot twist could be that Drago kidnaps Hiccup to use him as a bargaining chip against the dragons. Maybe he wants to force the dragons to submit to his will by threatening Hiccup's life. Another could be that Drago discovers some special power within Hiccup that he wants to harness for his own evil purposes. For example, Hiccup's ability to communicate with dragons might be something Drago wants to use to control all dragons.
One great drago x hiccup fanfiction is 'The Unlikely Bond'. It delves into how Drago and Hiccup might form an unexpected alliance. Another is 'Drago's Redemption', which shows Drago's journey towards changing his ways in relation to Hiccup. And 'A New Beginning' where they start to see each other in a different light.
There is also 'Hiccup and Astrid's Adventure'. This one takes them on a journey to a new land. The way it describes their interactions and how they rely on each other is really engaging. The author has a great writing style that makes you feel like you're right there with Hiccup and Astrid during their adventure.
I really like the fanfic 'A Different Kind of Friendship'. It starts with Hiccup and Snotlout being forced to work together on a project in Berk. As they spend more time together, they start to understand each other better. The writer does a good job of showing their growth as characters.
One great 'Female Hiccup' HTTYD fanfiction is 'A Different Perspective'. It offers a really interesting take on the story with a female Hiccup character. The author does a wonderful job of re - imagining the relationships and adventures in the HTTYD world.