Fire Dragon

the Fire Dragon was an ancient dragon-like being from Muspelheim which was involved in a battle with the Asgardian God of Thunder, Thor ,when he arrived to Muspelheim in order to confront with his master, Surtur.

Early Life
the Fire Dragon was born a millenia ago on Muspelheim and was created by the Fire Demon, Surtur, the ruler of the realm. Over the years, he used to live in Muspelheim and serve his master.

Battle of Muspelheim
During his confrontation with Surtur, Thor, which managed to release himself from his imprisonment by Surtur, he was freed and had spoken with Surtur about the oncoming Ragnarök prophecy ,which Surtur claims it cannot be stopped and needs to happen, presumably he knew about the threat of Hela.

When Surtur shown his malevolance and taunted Thor about the fact that the latter's father ,Odin is no longer on Asgard, the thing led to a lengthy battle on Muspelheim. During the battle, Surtur's creatures stormed on the Asgardian which used the Mjolnir hammer to defeat them all. In the process, when Surtur saw no option to overpower Thor ,he sent the Fire Dragon.

Death
When Thor called for Skurge to open up the portal of the Bifrost Bridge for him in order to allow himself to return to Asgard, the Fire Dragon had succeeded to come to the Bifrost Bridge with Thor himself, which after passing through the portal and returned to Asgard, had killed the monster moments after he arrived on Asgard by slicing the beast's head over the floor and destroyed it, causing the Dragon's yellow blood to be spilled over Skurge and the other Asgardian women which were with him.

Personality
Despite being only an intergal part of Muspelheim ,the Fire Dragon was a loyal beast for his master and creator, Surtur.

the Dragon was also a cruel and violent creature which known no limits than unleashing its wrath upon enemies, thus, the Dragon was able to reach to its full power and to kill his enemies easily.

Trivia

 * the Fire Dragon had no appearance in the comics alongside Surtur or Thor ,so the Dragon was an original creature created especially for the film.