The unnamed Fire Dragon is a minor antagonist of the 2017 Marvel film Thor Ragnarok. It is a giant dragon from Muspelheim that serves as the pet of the fire demon Surtur.
During his confrontation with Surtur about the impending event known as Ragnarok, Thor managed to release himself from his imprisonment by Surtur, who then summons his Fire Demons to take down Thor. Eventually, Thor used his hammer to take down the demons before defeating Surtur and taking his crown. Upon witnessing this, the angry Fire Dragon free itself from its chains, intending to kill Thor in revenge for its master's death.
When Thor called for the Bifrost Bridge to open in order to escape, the Fire Dragon managed to catch up with him. Needless to say, Skurge is currently the new Guardian of the Bridge and he was busy flirting with several Asgardian women until they gave him Thor's message. Without hesitation, Skurge managed to open the Bridge to let Thor return to Asgard, but this accidentally sliced the Fire Dragon's head over the floor, killing it and spilling its yellow blood all over Skurge and the Asgardian women, who left in a huff.
The fire dragon's behavior and mindset seemed to be reminiscent to a a loyal domesticated pet, as it display aggressive personality when Thor break free from his chains and attack its master, prompting it to enter the fight only to be hindered by chains that binding it. It is also very persistent, as whereas Surtur's minions seemed to lost Thor, the fire dragon did not give up the chase and even managed to catch up with Thor. Unfortunately, its blind pursuit to avenge its master led to its demise, as it accidentally beheading itself due to Bifrost beam that safely bringing Thor home, where the process caused its disembodied head ended up at Asgard along with the God of Thunder and the rest of its crippled body still at Muspelheim.