Tyrannosaurus rex

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A Tyrannosaurus rex fighting with an Apatosaurus. (DW: Invasion of the Dinosaurs

Tyrannosaurus rex (meaning tyrant lizard king) is one of the most famous dinosaurs. Tyrannosaurus rex hails from Earth, specifically what is now western North America. They lived towards the end of the Cretaceous. Although other theropods rivaled or exceeded T. rex in size, it was still one of the largest known land predators and thought to be one of the most dangerous predators on the planet. (DW: Invasion of the Dinosaurs)

The Tyrannosaurus was one of several biological specimens taken into cryogenic suspension by the Silurians. The Silurian shelter under Wenley Moor used a Tyrannosaur as a defensive measure, releasing it into the nearby cave system where it killed at least one human. (DW: Doctor Who and the Silurians)

During the 1970s invasion of London, the Tyrannosaurus rex was one of the dinosaurs that was brought into the present. The T. rex was one of the most dangerous creatures encountered by the UNIT troops. Though not very bright, they seemed to be bullet-proof and could only be scared away with grenades. One was even shown breaking through a wall. These T. rexes were large, ponderous creatures with short legs, lumpy features and a thick tail. The Third Doctor encounter at least 3 T. rexes and captures one in order to find out where they came from. (DW: Invasion of the Dinosaurs)

The Rani had several Tyrannosaurus rex embryos in her collection. These embryos were accidentally released after the Rani's TARDIS was modified by the Sixth Doctor. This embryo grew into a greener, more sleek T. rex , when compared to the previously seen ones. (DW: The Mark of the Rani)

The changes in appearance, from a real world point of view, was obviously due to changes in both special effects and knowledge. From the stand-point of the Doctor Who Universe, the differences may be due to different ages or being different species. Alternatively, the Rani's TARDIS may have affected the development of the embryo in The Mark of the Rani.

A zoo on an unnamed planet exhibited a tyrannosaurus or a tyrannosaurus-like creature. (TVC: The Didus Expedition)

The Tyrannosaurs was also native to the planet Raxas, and along with the other Raxas dinosaurs, proved to be more than a match for a Dalek attack. They were so strong they could bit the Dalek's dalekanium casings in half and crush at least four Daleks with their massive feet. (DWBIT: #7)