Smaug

“There he lay, a vast red-golden dragon, fast asleep; a thrumming came from his jaws and nostrils, and wisps of smoke, but his fires were low in slumber.”

The first thing Bilbo sees of Smaug, however, is the glow that lights up even the narrow secret passage. And when the dragon comes down at Esgaroth, the first thing the people see is the golden glow lighting up the river far away. This is why I made Smaug glow from within: he is the creature of fire, after all.