From the front of the D&D 5th Edition Monster Manual:

Disclaimer: Any similarities between monsters depicted in this book and monsters that actually exist are purely coincidental. That goes double for mind flayers, which absolutely, utterly, and completely do not exist, nor do they secretly run the D&D team. Do we really need a disclaimer to tell you that? You shouldn’t use your brain to consider such irrational thoughts. They only make the mind cluttered, confused, and unpleasantly chewy. A good brain is nice, tender, and rarely used. Go ahead, put down this book and watch some reality TV or internet cat videos. They’re really funny these days. You won’t regret it. We say this only because we love you and your juicy, succulent gamer brain.

An excellent book, starting right from the fine print page.