During the film's commentary, directors Chris Sanders and Dean DeBlois along with producer Bonnie Arnold reveal that Toothless's movements and body outline were based on one of the artist's cat. Toothless is the rare Night Fury, a dragon never seen by vikings due to its speed and camouflage in the night. He is dark black that helps him blend into the night sky and is sleek and agile for speed.
Toothless has wings like any other dragon and a tail with twin smaller wings for balance. Unfortunately, Toothless is injured in the beginning of the movie and loses his left tail-wing. His eyes are an intense green that could shift from docile to dangerous whenever the moment calls for it. Toothless was at first named due to his seemingly lack of teeth, but Hiccup later learns that his teeth are retractable. He apparently has ears, yet the film's commentary stated that they weren't ears but more of part of his body.
This Night Fury has four legs with sharp claws used for fighting or hunting. And lastly, Toothless has a noticeable snout like all the other dragons.
When the Dreamworks Animation logo first appears, you could catch a brief glimpse of Toothless flying behind the fishing kid on the moon. Then the actual movie starts with Hiccup's narration of Berk. When he describes the 'pests' of his home, a Night Fury snatches a sheep as the other dragons attack the village. In order to make his place among the Vikings, Hiccup builds a device that could down a dragon. He uses it during the raid and hits the Night Fury and lands in the forest.
To prove that he actually shot down the dragon, Hiccup ventures to the forest and find the Night Fury injured and tied up. Hiccup attempts to kill him but finds that he can't do it. Instead, he frees him, only to have it size him before roaring at him in warning and flying off. After learning from dragon training that all dragons go for the kill, Hiccup wonders why the Night Fury didn't and searches for him in the forest. He finds the dragon trapped in a cove without part of its tail wing, having him unable to simply fly out of the cove.
Seeing that he needs help, Hiccup decides to give him food and the two bond as Hiccup observes the Night Fury and vice versa. Hiccup names the dragon Toothless due to his retractable teeth and he begins to make a prosthetic tail-fin for him so he could survive and fly out on his own. However, this takes some time, during which Hiccup learns information about dragons that he uses to gain the position of top of his class. Eventually, Hiccup creates a makeshift tail-wing that he could use to direct Toothless as they fly together.
However, a girl named Astrid in Hiccup's class learns of the relationship and attempts to tell the village. In order for her to see that dragons aren't so bad, Toothless and Hiccup take her for a ride. She admits that it's pretty nice just as other dragons start flying around them. The teens learn where the dragons' nest is and that all dragons give up their food to a gigantic dragon that will eat them if the offering isn't satisfactory. The next day, Hiccup tries to show the Vikings that dragons are not that dangerous by refusing to fight a Monstrous Nightmare that he was assigned to kill in front of the village for being top of his class.
However, his father Stoick angrily attempts to stop the fight, upsetting the dragon who attacks Hiccup. Toothless hears Hiccup's cries for help which motivates himself to get out of the cove without any help. Toothless manages to arrive at the fighting ring and save Hiccup and defeat the Nightmare. The other Vikings attack him and Stoick has him locked up. Once Hiccup accidentally reveals that Toothless knows where the dragons' nest, it prompts Stoick to take an army of Vikings and a chained Toothless to the island.
When they arrive, the other dragons flee as the fearsome dragon attacks the Vikings. Thankfully, Hiccup, Astrid, and their classmates arrive to fight with dragons that Hiccup taught them to tame. As they battle the gigantic dragon, Hiccup and Stoick unchain Toothless. Soon, Hiccup and Toothless fight the dragon and eventually defeat him by setting him on fire that results in an explosion that knocks out and supposedly kills Hiccup. To the relief and happiness of Stoick, Astrid, the Vikings, and even the other dragons, Toothless manages to save Hiccup by protecting him in his wings.
Hiccup wakes up much later to find Toothless waiting eagerly for him and that his left foot was lost and has been replaced with a metal one. Also, Berk allows dragons into their village and Toothless and Hiccup ride around one last time as they are accompanied by Astrid and other classmates.