The Pteranodon is a minor alter ego of Calvin's. It appeared in a single Sunday strip, where Calvin read his book on dinosaurs and enacted a pteranodon fantasy by dressing up as one and jumping off the roof of his house in an attempt to fly.
The Pteranodon appears again in another fantasy of Calvin's, when he attempts to jump off the side of a hill and use his kite as a parachute. He describes the Pteranodon's "majestic" flight, and Hobbes likewise tells Calvin to "think majestic" in his attempt to fly.