Ziz is "the" giant bird at the end of the world in Christian mythology, to go along with Behemoth on land and Leviathan in the Mediterranean. God will slay these creatures so that the righteous will have food and clothing and tents, and possibly giant foam (leather) #1 fingers.
Actually, I believe Ziz comes from the Gnostic tradition and therefore it isn't in the Bible. But the more obscure the creature, the cooler it is to have in the game.
Cerberus allows the shades of mortals to enter Hades, but never to leave. I would imagine him as creature with 3 bite attacks that do 3x damage to undead, probably with 2 misc slots. Is there such a thing as undead-awe? That would be a great ability.