Calvin the Allosaur (identified as "Calvin, The Awful Allosaur") is a minor alter ego of Calvin's. It appeared in a single Sunday strip. In it, Calvin somehow transformed into an Allosaurus in his room and went outside to enter a Jurassic environment. There, it hunted a passing Diplodocus by means of ambush, as an analogy to Calvin scaring his mother by hiding in a closet and, upon her opening it, jumping out while making a hideous grimace.
Allosaurus appeared once more as a minor alter ego of Calvin's in a Sunday strip. In it, a Supersaurus (identified as an ultrasaur, in reference to Ultrasauros) took in a drink at a waterhole; the Allosaurus, also thirsty, tried to confront it for access to the water. However, as it approached its competitor, the Allosaurus realized it had no chance of bringing down the Supersaurus. Allosaurus was here analogous to Calvin waiting in line at a school water fountain to avoid being hit by Moe.
Allosaurus made one more appearance in a Sunday strip as a minor alter ego of Calvin's. In it, the Allosaurus ambushed a herd of Apatosaurus (identified as Brontosaurus) and jumped on a straggling member of the group. The Allosaurus was conscious of the danger of attacking such a large animal, but did so anyway due to desperation in hunger. Allosaurus was here analogous to Calvin jumping on his father while he was cooking, exclaiming that he was hungry.