I wonder how easy a defense would be if you just took a mindflayer and called it something different - can't exactly copyright the idea of a human with a squid for a head. Otherwise most depictions of Cthulhu would be in a heap of trouble.
I know they exist in a couple Final Fantasy games under the term "soulflayer," but I don't know if that's something they did with permission from Wizards of the Coast or not. (Though I imagine Square Enix wouldn't want to get in hot water with WotC again after what happened with the Beholder in the first FF game...)