Say a monster/NPC/whatever seeks a defense against reading his mind, but doesn't want to tip off would-be mind readers with a Ring of Mind-Shielding or something similar that just shields his thoughts. Instead, he wants them to believe they have successfully read his thoughts but in fact have gained misinformation. How can he do that?
I'm looking for existing ways to do this, not just making it doable with homebrew or DM fiat.