The Doctor may resemble Earth's dominent species (or rather, we resemble the Doctor), but if the Time Lord was traveling in prehistoric times, it would make more sense to blend with the dinosaurs. Dressed in the Doctor's finest fashions, these dinos turned out quite dapper.
There's a method to Peter Foglesong's madness in pairing certain lizards with certain Doctors. The First Doctor is a Megalosaurus because that was the first dinosaur discovered and recorded. The Fifth Doctor is a Pachycephalosaurus because that Doctor's wild hair and fondness for hats reminded him of the dinosaur's spiky skull. The Seventh Doctor is an Ankylosaurus because, well, that's his favorite doctor and favorite dinosaur as well. I'd like to think that the Brachiosaurus Third Doctor always had a cold neck, and that's why Number Four invested in such a long scarf.
Dino Who [Peter Foglesong via Design You Trust]