Stealth monsters are monsters which are normally invisible. They become briefly visible when they receive damage, fade out of visibility when calling A_Look, A_LookEx, A_Chase, A_ExtChase, A_FastChase, A_VileChase. They fade in, on the other hand, when calling A_FaceTarget, A_FaceTracer, A_FaceMaster, A_MonsterRail, A_CustomRailgun, or A_CPosAttack. (Note that practically all attack functions call A_FaceTarget.) Finally, the monsters become and remain visible when calling A_Fall, A_NoBlocking, or A_FreezeDeath.
Creating a stealth monster in ZDoom is easy: merely adding the STEALTH flag and giving it an Alpha of 0 is enough. Note, however, that when a stealth monster is visible, its render style will always be "normal". A fuzzy, shaded, additive, translucent, etc. stealth monster will not work; the stealth implementation directly changing the actor's render style.
A stealth spectre would be indistinguishable from a stealth demon, stealth bosses would be quite unfair, and the Easter eggs' presence in mod is generally frowned upon as an amateurish mistake.