The Tyrannosaurus rex was a pretty big creature... I'm not sure how it could be implemented into ESO. It also has to fit with the lore of the world and there are no known cases of T-Rex appearing in Nirn.
The closest you can get to a T-Rex in Elder Scrolls lore would probably be an Argonian Behemoth, so don't hold your breath. A wamasu mount on the other hand...