Apatosaurus

The Apatosaurus is the biggest dinosaur found in the valley of Eodon, being up to 120 feet long. This animal is actually no threat at all, apart from the fact that it could accidently step on the adventurer. They are vegetarians and mostly spend their time either in water or standing in swamp, where predators like the Allosaurus don't dare to enter.

Lore
''The Apatosaurus was for years known, and is still popularly known, as the Brontosaurus. Though by no means the largest dinosaur known in the outer world, the Apatosaurus is the largest to be seen within the Valley of Eodon, being some 80 to 120 feet long and weighing some 30-35 tons. Though prevailing scientific theory (and evidence within the Valley) holds that the apatosaurus does not actually spend the majority of its life in the water, the apatosaurus of the Valley does tend to seek the swamp-waters when danger is in the air; this terrain discourages the greater predators such as the allosaurus and tyrannosaurus. The native name for this beast translates, most appropriately, as "Longneck."''

-from "Ultimate Adventure Magazine", Savage Empire