The Hellhound, also known as the Cerebus, is an enemy in the original Final Fantasy. They appear as one of the most common enemies in the Gurgu Volcano, often appearing with Gigas type monsters. It is weak to Ice spell, as most of the enemies in the volcano, and absorbs the Fire spell. It may attack for low damage, or use Blaze, to hit all the allies for low damage (fire elemental). It is not hard alone, but as he appears in groups of more hounds, you may find out Ice spells spread out more, and they don't die very fast. Be persistent.
Hellhounds are related as the guardians of hell, which are portrayed as big demon dogs, with supernatural size and high speed an strength. As "Hell" is often imagined as a fiery place, hellhounds are often related to fire. Cerebus may be a typo from Cerberus, the biggest Hellhound, guardian of the gate of hell, represented as a big demon dog with three heads and fiery eyes. One thing to note is that, even if the GBA sprite looks as a red dog, the NES sprite portrays the Hellhound as a dog with a more human face, and horns instead of ears. Suggesting it is a dog from the depths of hell.