Brachiosaurus

The Brachiosaurus was a species of Dinosaur with a claim to being the largest such animal in history, according to the Fourth Doctor. Meeting one, he estimated that its head, at the top of its nearly vertical neck, must be about 12 metres from the ground. He also highlighted the peculiarty of its front legs being longer than its hind legs to his companion. (PROSE: The Doctor Who Dinosaur Book)