Nut

Nuts are dried seeds or fruits of different species of trees which appear in How to Train Your Dragon Franchise.

Description
The word nut may refer to a fruit with a hard shell and a seed (rarely two) or a variety of dried seeds used for culinary purposes.

The nut seen in the franchise is the common walnut which is the seed of the fruit called drupe produced by the trees in the genus Juglans.

Function
In both Franchise and reality, nuts are used as human food.

Dragons: Riders of Berk
In "When Lightning Strikes", Ruffnut and Tuffnut enjoy the destruction of the village while sitting on a roof and eating nuts by cracking them into each other's helmet.