The S that Snotlout draws on the ground is the actual Elder Futhark rune for the letter S.
Tuffnut's adoption of a chicken as a pet and his subsequent fierce loyalty to her is a reference to the classic computer game The Elder Scrolls V, which also depicts dragons and viking-themed characters. In that game, if the player kills a chicken, its entire village will attack in order to seek vengeance.
Introduction of the Night Terror dragon.
Introduction of Chicken.