Originally watchdogs of hell who came to our land to hunt down those who turned against the king of the underworld. They were charged with the duty of catching those disloyal to their king, and branding them with their fiery horns as a mark of their betrayal. The brand carried magicks of enforced servitude, and those so marked could never leave the underworld again. Though they no longer serve their ancient master, having long since been set adrift in our world, they continue to give chase and attack with fiery horns.
There is a flame fueled by vengeance and hatred, and never shall it flicker or fade. Perhaps it is the hell-gate's flame burning on the horn of the cerberus that has earned it the title 'Watchdog of Hell'. Yet these flames touch none but the wicked, and singe the good not in the least. Hence the custom of searing the sinful with flame to adjudge the weight of their sins.
Cerberus can be easily taken down with regular attacks, unless the party is at low levels.
If one is trying to get the Zodiac Spear as soon as possible and pass through Feywood, one should simply flee unless lucky enough to obtain a Nihopalaoa; with conjunction to one using a Phoenix Down instantly defeats them.