The in-game information on Tiamat is limited to the description you get when you use Scan on him:

"Used to be a GF. Became a monster under Ultimecia's power. Its Dark Flare destroys all enemies."

There was a discussion a while back on whether Tiamat was a future version of Bahamut, since they have the same character designs (with different colors). It's unlikely since Tiamat and Bahamut have traditionally been separate characters in other FFs.