Bahamut is the King of the Dragons in the original Final Fantasy. He lives in a subterranean network underneath the Cardia Islands, and grants the class change to the Warriors of Light after they bring back a token of their courage, the Rat Tail, from the Citadel of Trials.
In the Soul of Chaos, there is a Dark Bahamut in the Lifespring Grotto. Like the version that changes the player characters' class, Dark Bahamut sets the party on a trial. Dark Bahamut's trial is to slay a certain number of specific colors of dragon from his underlings, and the reward is that he will open the door and let the party out of the room. The quest can be completed by killing all of the dragons in the room without concern for their color.
Bahamut (Arabic بهموت Bahamūt) originated as an enormous whale in ancient pre-Islamic Arabian mythology. Upon Bahamut's back stands a bull with four thousand eyes, ears, noses, mouths, tongues and feet called Kujuta (also spelled "Kuyutha"). Between each of these is a distance of a 500-year journey. On the back of Kujuta is a mountain of ruby. Atop this mountain is an angel who carries six hells, earth, and seven heavens on its shoulders.
Another version of the Arabic story is that Bahamut is indeed a dragon and he stands on a whale called Liwash.
In modern times, the game Dungeons & Dragons is responsible for re-imagining Bahamut as the king of dragons, a benevolent Platinum Dragon; the opposite of the malevolent Tiamat, the five-headed Chromatic Queen of Dragons.