The dragon killed by Túrin in J.R.R. Tolkien’s legendarium is Glaurung.
Glaurung is known as the Father of Dragons and appears prominently in the First Age stories later collected in The Silmarillion and The Children of Húrin. He serves Morgoth and possesses intelligence, speech, and a powerful hypnotic gaze.
Túrin Turambar eventually confronts Glaurung at the ravine of Cabed-en-Aras. The dragon is vulnerable from below, and Túrin kills him by driving his sword Gurthang into his belly. Glaurung’s death does not end the tragedy surrounding Túrin, because the dragon has already manipulated Túrin’s family through deception and enchantment.
Glaurung is distinct from Smaug, the dragon in The Hobbit, and Ancalagon the Black, another great dragon of the First Age. The dragon episode belongs to the much older and darker history of Beleriand, long before the events of The Lord of the Rings.