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.

Major Baker was attacked and wounded by the creature after he fired upon a Silurian he had seen. (TV: Doctor Who and the Silurians)

Behind the scenes
In the novelisation, the creature is a Tyrannosaurus rex. Although the dinosaur is never identified on-screen, it does indeed appear to be a tyrannosauroid of some description. The fact the Doctor can not identify the species suggests that it may in fact be something genetically engineered by the Silurians that did not coexist with natural Dinosaurs.

The dinosaur featured in Doctor Who and the Silurians was made by visual effects assistant Anna Baybrooke, and operated by visual effects assistant Bertram A Collacott. The character was listed as "Bertram, the Friendly Monster" on script cast lists, played by "himself". (REF: Who-ology: The Official Miscellany)