Hiccthur

Hiccthur is the poly ship between Hiccup Horrendous Haddock III, Dagur the Deranged and Heather from the How to Train Your Dragon fandom.

Season One
Shortly after Hiccup saves Heather from Dagur, she tells him about what she remembers of her birth father before ended up in the care of her adoptive family; who have been killed by Dagur and his forces. Which is why Heather has been hunting Dagur down so she could avenge them, along with the lives of her fellow villagers. As Heather told Hiccup this, she had showed him the horn that was a gift from her birth father; but when Hiccup got a closer look at it he saw that his father's chief seal is craved on it. Putting two and two together in his head, Hiccup flies back to Berk so he could hopefully get Stoick to conform his suspicion of Heather (possibly) being his sister. Stoick explains that while the horn is a gift from him to Heather, however, it wasn't because he is Heather's father but to simply honour the tradition of presenting a fellow chief's new born child with a gift; and while the said child was in fact Heather her birth the chief father was nonother than Oswald the Agreeable.

Realizing that Heather is Oswald's daughter, as well as Dagur's younger sister, the one person that she seeks vengeance upon, Hiccup races back to the Edge in order to tell her this before she does anything that she'll later regret. While Hiccup is able to stop Heather from killing her brother, just before telling her of what he had learned about the origins behind her horn, she is horrified to know that she shares blood with the one who killed her adoptive family. After breaking free from his bonds, Dagur jocks about Hiccup possibly being his "uncle" instead of being his "brother", before he tries to convince Heather to join him as a brother-sister duo; to which Heather turns him down on before she flies away with Hiccup and the rest of the Dragon Riders. As she still hates Dagur for what he did to her. Understanding that Heather will need sometime alone to suggest such news, as well as knowledge, Hiccup sees her off.

Moments

 * Dagur calls Hiccup "our" brother in his letter to Heather.
 * Before Hiccup and Heather go to help Dagur, Hiccup calls him "our" brother.

Fanon
Because of Dagur countlessly calling and referencing Hiccup as "his brother", along with Hiccup's question to Stoick about Heather being his "sister", due to him finding his father's chief seal on Heather's horn, just before it was reviled that she is in fact Dagur's long lost sister, a few fans began to wonder what it would have been like if all three of them are siblings; with Hiccup and Heather as twins while Dagur is their older brother. Since all three of them have green colored eyes and are shown to be "deadly" when their anger is pushed to the limit, some fanon alterations of this How to Train Your Dragon "what if", however, doesn't always have Hiccup and Heather related to Dagur at all. As well as the reason why the poly ship is sometimes treated as a familyship.

Fandom
FAN FICTION

Variations

 * Dagcup refers to the ship between Dagur and Hiccup
 * Dagther refers to the ship between Dagur and Heather
 * Heatcup refers to the ship between Heather and Hiccup