Hiccup Haddock II is the Great Grand Uncle of Hiccup and Snotlout. Haddock II had a interest in dragons much like Hiccup. 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 is also mentioned by the witch, Alvin's mother, in How to Break a Dragon's Heart.

Haddock II was abandoned by Grimbeard on a mountainside for being a runt. He was found by dragons brought up as one as well as his adopted brother Furious a seadragonus gigantus maximus. After he was reunited with Grimbeard, he organised a peaceful dragon rebellion to show Grimbeard the truth about dragons. Thugheart however tricked Grimbeard into believing that the dragons had violent intentions. His men fought the dragons, and he killed Haddock II.