A creature, or entire race of creatures, can only be seen by individuals who possess a certain trait. For anyone who does not share that trait, the creature cannot be perceived at all. Usually, losing the trait will also cause you to lose the ability to perceive them, though in some cases, once you have encountered the creature, you will always be able to perceive them. This is the supertrope to Invisible to Normals and Invisible to Adults. Please place examples of those subtropes under the proper subtrope. See also Weirdness Censor, Magnetic Medium, and You Can See Me?.