The Hydrus (also known as Enhydros, Enidros, Hildris, Hydra, Idra, Idres, Ydre, Ydris, and Ydrus) is a creature from Medieval bestiaries. They were said to be found in the Nile River. While in the Nile, a crocodile would roll the hydrus in the mud before eating the smaller creature. However, once inside the crocodile's stomach, the hydrus would burst free from the stomach lining. Another interpretation is that the hydrus would intentionally roll in the mud and seek out the crocodile while it slept with its mouth open.[1] In medieval Christian tradition, this bursting free from the crocodile became an emblem of the resurrection of Christ bursting free from Hell.[2]
A hydrus has no set appearance. Normally it is considered to be a water snake, although Pliny the Elder believed that it was an otter. Other animals the hydrus has been thought of as include a bird, a dragon, and a mongoose.[3]