T-Rex seems less rare because we spend more time navigating the Mirage Desert (It may have a different name, but I don't recall it) than we do the Wind Room of the ToF.

Both encounters are ridiculously rare, and I have discovered that they have correlating rare encounters in other parts of the world (though nobody mentions them because those rare encounters are simply oddly formed enemy parties that we slay along with the rest without a thought. The best example I know is a party of scorpions in Elfland or a 2 Ogre/Green ogre party in the plains near the elfland town (not in the forests near Elfland or the area by the Marsh Cave).

If you fight 255 battles near elfland, chances are fair (not nearly certain) you'll find those scorps or ogres (but you won't fight THAT many battles).
Same for the Desert and T Rex.
You'll probably get into about 6-8 random encounters in the Wind Temple of the ToF. Chances of seeing a Iron Gol: Laughable.