Here's an informative guide to the most fascinating prehistoric creatures that ever walked the planet-from the three-horned Triceratops and leaf-eating Ornithopods to egg-stealing Oviraptors and the dreaded Tyranosaurus. These fascinating creatures (and many others) are brought to vivid life by famed dinosaur illustrator Robert Walters, who helped Steven Spielberg design the dinosaurs for the phenomenally realistic film Jurassic Park , and whose work has appeared in National Geographic, Discover, and other national magazines.