Hiccup Horrendous Haddock I is the ancestor of Hiccup and Snotface Snotlout. He was the first King of the Wilderwest, although he is also blamed for the Hooligan Tribe losing their land on the Mainland and being forced to move onto the little Isle of Berk. He lived over 1,000 years before Hiccup Horrendous Haddock III.
Like all Hiccups in the Hooligan Tribe, he was so named because he was a runt, an accident, a hiccup. Like the other Hiccups, he was fated to change history.
Hiccup Horrendous Haddock I's story strongly parallels that of the films' Hiccup. The Vikings and Dragons had been at war for a long time before Hiccup was born. Hiccup was targeted by the leader of the dragon army, a great Sea Dragon named Merciless (later known as the Green Death), who had forsaw that one day he would be killed by a boy named Hiccup. The dragon he sent to kill Hiccup was the Wodensfang, whom Hiccup befriended. The Wodensfang had gotten caught in a tree and was severely injured, but instead of finishing him off Hiccup the First cut him loose and healed his wounds, cultivating a bond of trust between the two. The Wodensfang taught Hiccup Dragonese, and he in turn taught the Wodensfang Norse. Hiccup the First became the first human to ride on a dragon's back.
Hiccup the First was given the Dragon Jewel by the Wodensfang, and in return was given the first Dragonmark, which Hiccup the First then promised the Wodensfang that by that mark, he would not use the gift for evil. He ended the war between dragon and humankind by threatening to use the jewel's awful power. Thereafter the Viking Tribes joined with many of the dragons as equals, training hunting dragons and riding dragons. The dragons were fed first at every hunt, sat around the fire with the men and gave their opinions before attacks. As for Merciless himself, Hiccup made him live again as a loner in the deep ocean and warned Merciless never to return to the Archipelago. For a long time Hiccup's descendants kept the promise he had made to the Wodensfang, until Grimbeard the Ghastly broke it and began to enslave dragons and humans.
Only one picture of Hiccup The First appears in the series; and that's in The Complete Book Of Dragons. In the picture, he is riding on a silver phantom and has a similar appearance to Hiccup (Franchise). His hair is quite scruffy, he's wearing a firesuit and the face is the same shape.