A tyrannosaurus rex would be interesting to see, but I do agree that they at the very least seem to be holding off on it (if not avoiding it).
I believe Tyranitar is a Tyrannosaurus Rex. He looks a bit like one, and even has the "Tyran" in the name.
Also, bulbapedia says this: "Tyranitar seems to be based on Godzilla, a kaiju known for its strength, tough skin and ability to shoot beams from its mouth. Godzilla itself was designed by combining three dinosaurs: Iguanodon, Stegosaurus and Tyrannosaurus."