The Spidasaur is an aquatic beast native to Eternia. It resembles an enormous red serpent with six tentacles.
He-Man and his friends first encountered a Spidasaur in the Region of Flame while they were on their way to meet with the Fire People. It emerged from a lava pool and seized the Attak Trak in its jaws, but they used the vehicle's electric defense mechanism to escape. King Helios was also caught in a Spidasaur's tentacles during his fight with He-Man, and would have been eaten by it had He-Man not thrown a boulder into the beast's mouth.
Another such beast appeared in a later episode, emerging from a desert oasis. This suggests that Spidasaurs are very adaptable and can survive in liquids of many different temperatures. Aside from a slightly different coloration, it was identical to the one(s) from the Region of Flame.
- This creature was called a "Serpent" in the production script for "The Curse of the Spellstone." It was not given the name "Spidasaur" until it made its next appearance.
- The Spidasaur looks similar to the cistern fiend, an aquatic monster from the world of Athas in Dungeons & Dragons.