Mythological Creatures

Mesuza: Spawning in caves deep in the ocean. If someone (including animals and monsters) look at her for too long, they freeze and become gray. They start to sink and create maybe a new kind of stone or something else that you might want.

Cerberus: He may spawn only when the world is first created (like the villagers and the ender dragon whom spawn once and can't spawn again) near broken portals, or maybe somewhere in the nether.

What will he protect? I think broken portals should have something very valuable and good on the other side if you open them (And only then. Otherwise you could just go to the nether and randomly find one of these). Cerberus is going to try to prevent you from achieving that very important thing.

It's going to add a lot more interest and challenge to the broken portals and would add to the feeling of the nether as a very dangerous place with lots of adventures.