Rex the Wonder Unicorn was not only Princess Melithiel's pet, but a beautiful were-unicorn which sometimes had wings and sometimes did not. He served many functions in the Great Underground Empire. In five minutes he could change from his unicorn-self into a cute elf maiden who spoke in couplets or a winged vampire or anything. And he had a crystal horn that glowed in the dark. While the unicorn did not seem any larger or stronger than an average-sized horse, it pulled the princess' large chariot, not only on land, but lifting it off the ground for aerial treks. Like most normal unicorns, a delicate gold key hung from Rex's neck by a red satin ribbon

During spans of time when Melithiel was kidnapped and guarded by various dragons, Rex waited for her in the nearby garden, where he would crop grass and eating roses in boredom. He would shy away from all others, letting none draw near but her. When the princess was rescued by the Second Dungeon Master in 948 GUE, she presented him, as a reward, the were-unicorn's delicate gold key. Rex, harnessed to Melithiel's chariot, carried Mirakles and his companions out of the depths of the Dungeon of Zork after they had defeated the Autoexec.