The Calvinosaurus is a theropod dinosaur discovered by Calvin. Its first appearance was in Calvin's imagination. In it, it was said to be so large that it can "eat an Ultrasaur in a single bite".
Much later on, during show-and-tell, Calvin displayed a fragment of jawbone fossil he found in his front yard. He realized it belonged to a new dinosaur species. He also produced a dramatic illustration of the complete Calvinosaurus, which was very large, not too much unlike the version that appeared in his imagination. For reasons unknown, his demonstration had him sent to Principal Spittle. Curiously, Calvin claimed that he would be publishing his findings shortly, yet there is no proof that he followed through with this.
He appeared in real life while being popped out of Calvin's brain in Calvin and Hobbes. He occasionally takes a nap, for example resting on Free Cape Included. When traveling with Calvin, Hobbes, Magical Tiger, Avery Elmhurst, Zoe, Stupendous Man, Tracer Bullet, and Spaceman Spiff in his ship, he is too big to fit inside the ship, so they duct-tape him to the top. In the first draft of the same film, he was created to protect Avery when they went to Isla de Sombra. He was friendly and highly intelligent for a dinosaur, and managed to defeat Megamecha. In the fan-made sequel, he has become Calvin and Avery's pet.
- Fragments of Calvinosaurus jawbone resemble driveway gravel to the ordinary layman.
- When Calvin was unearthing garbage he mistook for dinosaur bones, he intended to call the complete skeleton a Calvinosaur.
- Ultrasaurs can "weigh over 70 tons" and are bigger than apatosaurs, so the Calvinosaurus is monumentally large.
- The Calvinosaurus resembles a Giganotosaurus.