The Piscodemon, also known as the Wizard in the NES version, is an enemy in the original Final Fantasy. It is known for guarding Astos'crown, as well as other locations:
The space within the lowest left-hand door in the third basement, Marsh Cave.
Left side of a chest, found in the second room from the bottom, counting from the left. Marsh Cave, basement 3.
Guarding a chest in the lower right hand corner. Marsh Cave, basement 3.
They are tough enemies when encountered for the first time as their attacks inflict a large amount of damage and they are impossible to escape from. If you have a Black Mage that has learned Thundara from the Elfheim Magic Shop, there is little to worry about.