Ancalagon, the Black

Ancalagon was unleashed by MORGOTH in response to the intervention of the Valar in the conflict in Middle Earth which became known as the WAR OF WRATH.  
His appearance along with a fleet of lesser beasts was devastating.  His assaults were concentrated on the Dwarven contingent in the army of the Elves and their allies.  Many Dwarves died that day and this probably started the enmity between these two groups of creatures.  
He was eventually killed, but some of his offspring made their way into the NORTHERN WASTES, and still appear periodically to plague Middle-earth to this day.