Hookfang is a male Monstruos Nightmare owned by Snotlout.
Kept in captivity with the other dragons, he wasn't used for the training but he was the prize for the first of the class, who had the honor to kill him. Hiccup was the one chosen but decided to not kill him and, instead, he shown the Vikings that dragons are not bad. Unfortunately, after Stoick shouted to his son, Hookfang was scared and started to attack Hiccup, who was saved by Toothless. Hiccup gave Hookfang to ride to Snotlout, who was very scared at the beginning.
In the TV series, Hookfang rarely obeys his rider. Deep inside he cares but he doesn't really stand the childish behaviour of Snotlout, which makes Hookfang act with disrespect.