Hiccup Haddock II is the Great Grand Uncle of Hiccup and Snotlout. Haddock II had a interest in dragons much like Hiccup III. In A Hero's Guide to Deadly Dragons, Hiccup finds a book entitled 'A Hero's Guide to Deadly Dragons', written by Hiccup Horrendous Haddock II. This goes against Stoick's earlier saying, "Horrendous Haddocks don't write books."
Haddock II was abandoned by Grimbeard on a mountainside for being a runt, after which his mother Chinhilda set out on a years long quest to find him, eventually disappearing in the Bay of the Broken Heart. He was found by dragons and brought up as one with his adopted brother Furious, a Seadragonus Giganticus Maximus. After he was reunited with Grimbeard he quickly became his favorite son, despite their tremendous differences. Hiccup organized a peaceful dragon protest to show Grimbeard the truth about dragons, but Thugheart tricked Grimbeard into believing that the dragons had violent, rebellious intentions. Grimbeard instantly smote Hiccup at the dragon's arrival, leaving a bloodstain on the Throne of the Wilderwest that remains even by the time of Hiccup III. Though he quickly realized he had been tricked, the damage was done: an extremely bloody battle had taken place and his son was dead. Realizing the truth of Hiccup II's intentions caused Grimbeard to be overwhelmed by grief, banishing Thugheart and dissolving the Kingdom of the Wilderwest. Furious buried Hiccup in the Bay of the Broken Heart, where he stayed to mourn till he was caught and imprisoned on Berserk. The events surrounding Hiccup Horrendous II's death directly led to the events of the novels.