Diplodocus

Diplodocus was a 30 meter long, 15 ton sauropod dinosaur from the Late Jurassic time period (155 - 145 MYA) that lived on the territory of the modern Western United States. Other potential Diplodocus specimens,  as Seismosaurus, were 15 meters longer and weighed nearly 7 times as much, making Diplodocus perhaps the biggest dinosaur on Earth.

Diplodocus was one of the longest animals ever to live. Seismosaurus, a species of Diplodocus, was 39-53.9 meters long (45 meters on average) and possibly over 100 tonnes in weight making it the longest animal that has ever existed and possibly the largest land animal. In fact, it was so large that even a pack of allosaurs wasn't able to hunt fully grown specimens (though they would attack individuals which are weak, sick, old, injured and young). They lived in huge herds, sometimes up to 100 dinosaurs, led by a matriarch, as the modern elephants do.

Diplodocus may had a long lifespan sometimes up to 120 years (it could be shorter lifespan if it is a warm-blooded animal like most dinosaurs). Diplodocus hatchlings ate 2-3 kilograms(4-6 pounds) per day, until they reached young adulthood.

Diplodocus probably lived in a symbiotic relationship with a small pterosaur called Anurognathus that cleaned ticks and other parasites that lived on skin of Diplodocus. It should be noted that while it doesn't appear in Prehistoric Park, a stylized drawing of this animal is on logo of Prehistoric Park. However, this sauropod dinosaur was featured only in the second episode of Walking with Dinosaurs and in Walking with Dinosaurs Allosaurus, a Walking with Dinosaurs special. It was also featured in cameos in the beginning of Walking with Beasts and the end of Walking with Monsters (both times unidentified).