Newtsbreath is a Gronckle, which is a tough, though ugly species of dragon.
Newtsbreath and Hookfang are in the chief's hut when Toothless wakes up. The two dragons greet Toothless and Hookfang swipes at Toothless, causing the little green dragon to howl in pain. Stoick then yelled at both hunting dragons to get out.
Newstbreath appears again in Book 2. Newtsbreath is revealed to not like baths as shown when he and the rest of the Hooligans get baths before going to the Isle of Skullions to search for the Treasure of Grimbeard the Ghastly.
Newtsbreath somewhat fears Stoick due to the fact that he is bigger than him. Because of this Newtsbreath obeys Stoick when he yells at him.
Newtsbreath is prone to bully Toothless as he doesn't like him.
Newtsbreath appears to respect Hookfang in a sense and appears to be his friend also in a sense.