Apatosaurus

Apatosaurus, often incorrectly referred to as Brontosaurus, was a type of sauropod dinosaur on prehistoric Earth. In plain English, the creature could also be referred to as an Apatosaur.

Biology
They were big, bulky, vegetarians with incredibly long necks. (TV: Invasion of the Dinosaurs) They were 20 metres long and, not counting the neck, four and a half metres high. Apatosaurus lived mostly on weeds and plants found in and around lakes; being submerged in water for most of its daily life helped the Apatosaurus deal with its own enormous weight. Despite "looking a bit dim", Apatosaurus was actually a fairly clever animal, with the Fourth Doctor opining that it might have made a good pet if not for its size; according to the Doctor, an Apatosaurus had two brains, one in its skull and one at the end of its spinal cords, at the top of the tail. (PROSE: The Doctor Who Dinosaur Book)

History
A few Apatosaurs were briefly transported to 1970s-era London by Operation Golden Age. (TV: Invasion of the Dinosaurs) This was mentioned by the Fourth Doctor following his regeneration. (TV: Robot)

The Fourth Doctor later travelled back in time to the Age of the Dinosaurs to show a human the time in Earth's life when it had been populated by "monsters" as fearsome as any found on alien planets. After the Doctor fed a friendly Apatosaurus some lake-weed, however, the time-travellers witnessed it being attacked by an Allosaurus which pounced onto the Apatosaurus and bit at its neck while ripping its prey's flesh with its claws. (PROSE: The Doctor Who Dinosaur Book)

Behind the scenes

 * According to The Battle for Raxas, a short story published in DWBIT 7 which is not considered valid on this Wiki, the Apatosaurus was also native to the planet Raxas. Unlike most of the other dinosaur species on Raxas, they were rather non-violent.