In the movie, Toothless is a massive, jet black dragon. On Toothless tail he has a set of tail fins. He lost his left one when Hiccup shot him out of the sky. Later, Hiccup makes him a new mechanical tail fin that he uses to fly. Toothless's eyes are a yellow and green mixture. When he is happy, his pupils tend to become larger and rounder. When he is angry, his pupils tend to turn into narrow slits. He is animated in CGI.
Before Toothless was trained he was often times aggressive. He attack Vikings and other people. After being trained Toothless started to open up to new people. Toothless's favorite person to hangout with is Hiccup. Toothless also likes Astrid, the other dragon trainers, and the other Vikings.