This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Hydrus (or Hydros) (Medieval Bestiary)
The Hydrus 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.
In medieval Christian tradition, this bursting free from the crocodile became an emblem of the resurrection of Christ bursting free from Hell. The hydrus was and is also sometimes confused with the Hydra of the Hercules legend, with some texts saying that it was a many-headed water dragon, living in the swamp of Lerna, that could grow new heads.