The fire-drakes; Dragons who breathed fire. Glaurung, Father of Dragons, was the first of these. He first appeared in the middle of the First Age, but the urulóki certainly survived the downfall of their master, Morgoth.
Ancalagon apparently belonged to this kind, and Smaug, the great fire-breathing dragon that sacked Erebor, seems to have been the last of the great urulóki. Other lesser types apparently survived to the end of the Third Age and beyond.