—In-game description in The Sacred Stones
The Gwyllgi (ケルベロス Keruberosu, lit. Cerberus in the Japanese version) is an enemy-only Monster class that is exclusive to Fire Emblem: The Sacred Stones. The promoted form of the Mauthe Doog class, these savage, three-headed hellhounds are significantly more dangerous than their unpromoted counterparts, ripping the flesh of their foes with deadly attacks administered by their fangs. Gwyllgi are distinguishable by their three-headed forms, alongside a mane that is murky green in colour.
Gwyllgi often possess high Skill and Speed, as well as decent Strength, making them particularly suited to killing slower, weaker units. Their maximum stats are equal to those of a male Swordmaster, but their bases and growths are superior. Furthermore, they have a Movement range that is equal to that of a Paladin. Most, if not all Gwyllgi, are equipped with Hellfangs, which are the same as Silver Swords.
Due to their high Speed and Skill, they are some of the more accurate monsters, and very few units are capable of doubling them. While their defenses are still poor like their unpromoted counterparts, they have enough defense and health to shrug off an attack that isn't a legendary weapon. Because of this, units with high enough Defense to shrug off Gwyllgi attacks are recommended to be deployed when faced with such creatures on the battlefield.
"Gwyllgi" means either "Wild Dog" or "Twilight Dog" in Welsh. "Cerberus", the Japanese name of the class, is derived from the creature of the same name from Greek and Roman mythology.