The Ologhai, also known as the Olog-Hai, is an enemy from Final Fantasy III. Like most enemies in the Sunken Cave, it is Water-based and thus weak to Lightning. They are somewhat powerful with their physical attacks, but are of no big threat to the party when fought.
"Olog-hai" is the Black Speech name for trolls in J.R.R. Tolkien's fictional mythology of Middle-earth. More specifically, it refers to a special breed of 'super trolls' who were invulnerable to sunlight, the one weakness of their lesser brethren.