Playing on survivial mod, on difficulty of easy, normal and hard, I would set out with my sword and armor to hunt mobs.

I would here the zombie noises, not see them though, just hear them. They would still harm you though. They would burn in the daylight, take fall damage, but you could not see them flames or the hitbox. Upon death they would drop the normal loot, and die normally, though invisibly.

This has happened to me in Creative and Survivial mode alike. They do not spawn from a spawn egg, only naturaly. If I log out and log back in again, they are visible again.