Tolkien designed his own
taxonomic system for dragons, based on locomotion and fire-breathing.
Some dragons (Glaurung) walked on four legs, like
Komodo dragons or other lizards. Other dragons (Ancalagon,
Smaug) could both walk on four legs and fly using wings. Winged-dragons were only first witnessed during the
War of Wrath, the battle that ended the First Age, so all dragons seen before the end of the First Age could not fly (such as Glaurung), although breeds of wingless dragons did exist.
Dragons who could breathe fire were called
Urulóki (singular
Urulokë), "Fire-drakes". It is not entirely clear whether the term "Urulóki" referred only to the first dragons such as Glaurung that could breathe fire but were wingless, or to any dragon that could breathe fire, and thus include Smaug. In Appendix A of
The Lord of the Rings Tolkien mentions a "Cold-drake". It is commonly assumed, though not directly stated, that this term indicated a dragon which could not breathe fire, rather than one who "breathed" ice or snow like the White Dragons of
Dungeons & Dragons. Further, Tolkien calls a fire-breathing dragon in the non-Middle-earth story
Farmer Giles of Ham a "hot" one.