He was created when the god Io was killed in the Dawn War. Io was cleaved in half by the primordial Erek-Hus, and when his body fell, his two halves arose as two new gods. His left half took on Io's noble traits of compassion, sense of justice, and love for his creations, and became Bahamut.
Bahamut appears as a great wyrm with shining silver-white scales. However, he appreciates subtlety, and when he chooses to appear among mortals, he often does so in the guise of an old man followed by seven trained canaries, which are, in truth, a flight of gold dragons who are Bahamut's closest servants.