Gjallarhorn

"The last stopping point on the map is the wreck of the Gjallarhorn. You'll know it when you see it-- it's the giant shipwreck that's fifty feet up in the air!"

- Hiccup

Gjallarhorn is a Viking Longship that shipwrecked at Helheim's Gate prior to the events of the first film.

History
During one of the Hooligans' search parties for Dragon Island, Gjallarhorn shipwrecked in Helheim's Gate and ended up hanged on a rock formation, fifty feet above the water level.

When Stoick the Vast and Berk's entire army went to Dragon Island guided by Toothless, they passed by the remains of Gjallarhorn.

Years later, when Hiccup tried to find Dragon Island, he used Gjallarhorn as a stopping point on his map, as he remember that the wrecked ship is very close to Dragon Island.