Mind flayers, also known as illithids, are one of Dungeons & Dragons' most iconic original monsters. They're psionic, imperialist humanoids who can be found from the darkest recesses of the Underdark to the farthest regions of Wildspace. Illithids have been a Dungeons & Dragons staple since the game's very first iteration.
Source:https://www.cbr.com/dnd-mind-flayer-history-tactics-campaign/