Cirein-cròin was a giant sea monster featured in Scottish folklore. According to legend, it was the largest living creature, and could eat seven whales, with the Devil himself being the only creature that would be able to slay Cirein-cròin. It was also capable of shapeshifting, as when a fisherman neared the monster, it would turn into a small fish, which would be caught. Cirein-cròin would then return to its true form and eat the fisherman.
According to writings by Alexander Forbes, the nature of Cirein-cròin is unknown, but it was widely believed to have been a sea serpent, although he proposed it as an Atlantosaurus.