The Silenus, also known as the Sirenos, is an enemy from Final Fantasy III that can be encountered in the Ancient Ruins. It will split in two if he is attacked by a weapon other than a Dark Sword or when not inflicted with Paralysis. For this reason alone, it would be highly recommended to bring along a couple Ancient Swords, bought in these ruins in the NES version, to help with dealing with these enemies. If it is attacked with magic, it will not split in two.
In Greek mythology, Silenus was a companion and tutor to the wine god Dionysus.