Ancalagon the blackAncalagon the Black - The great dragon of Morgoth at the end of the First Age.  Ancalagon led a host of dragons against the Valar in the War of Wrath.  He was eventually slain by Eärendil.

 

"Then, seeing that his hosts were overthrown and his power dispersed, Morgoth quailed, and he dared not come forth himself.  But he loosed upon his foes the last desperate assault that he had prepared, and out of the pits of Angband there issued the winged dragons, that had not before been seen; and so sudden and ruinous was the onset of that dreadful fleet that the host of the Valar was driven back, for the coming of the dragons was with great thunder, and lightning, and a tempest of fire.

 

"But Eärendil came, shining with white flame, and about Vingilot were gathered all the great birds of heaven and Thorondor was their captain, and there was battle in the air all the day and through a dark night of doubt.  Before the rising of the sun Eärendil slew Ancalagon the Black, the mightiest of the dragon-host, and cast him from the sky, and he fell upon the towers of Thangorodrim, and they were broken in his ruin.  The the sun rose, and the host of the Valar prevailed, and well-nigh all the dragons were destroyed..." ~The Silmarillion, chap. 24 (The War of Wrath)