Silurian pet

The Silurian pet was a member of a species of dinosaur unlike any the Third Doctor had seen before. It was preserved from extinction by the Silurians and revived with them in the 20th century, where it guarded the caves underneath Wenley Moor. (TV: Doctor Who and the Silurians)

Behind the scenes
In the novelisation, the creature is a Tyrannosaurus rex.