Freya

Freya is the Norse goddess of fire, love, beauty, and fertility. She is mentioned sveral times throughout the How to Train Your Dragon books and franchise.

Portrait of Hiccup as a Buff Young Man
The Dragon Riders are trapped inside an ice cave trying to solve one of the clues to find the treasure of Hamish II. The clue states: "Call on Magni; you'll go astray. Freya, though, will show the way."

- Third Clue . Snoutlout then partly breaks the ice, setting of a trap that nearly kills him. Fishlegs and Hiccup then realize that the clue is telling them to use fire rather than strength, because Magni is the god of strenghth and Freya is the goddess of fire.

Trivia
In the actual Norse mythology, Freya isn't the Goddess of Fire, she is instead, the Goddess of Love, Beauty and Fertility.

External Link
Freya on Wikipedia