Actually Tiamat does exist, but he is very hard to encounter.
First you have to defeat Intangir and get him to drop the "Mysterious Gem" (1/64 chance). Once you have the Mysterious Gem take it to the crazy man by the Veldt, the house where Sabin washes up when you first start his journey to get back to Narshe. Talk to the man and he will mention that the Mysterious Gem belongs to a fierce dragon living on the Veldt. Now you can go to the Veldt and fight random encounters, and he will eventually show up. He is a really hard boss, especially for the WoB, but beating him nets you an Economizer.
