I cannot find the Illithid (Mind flayer), or certain dragon variants in the SRD. Now, I know that Yuan-ti are off limits.. but I didn't think the Mind Flayer was.

In light of my discovery, should I just assume that Mind flayers are not Open Gaming Content?

This really is Strange. I thought I saw a 3rd party book on mind flayers.. hmmm

Any help sorting this out would be most appreciated.

Hi, DavidJ. Good to see you here; I was a big fan of Bauhaus and Tones on Tail.

Anyway, yes, Wizards thinks they own the Mind Flayer as IP. The Slayer's Guide to Mind Flayers just beat the deadline for their removal of those creatures from the public domain, as it were.

I don't know how enforceable those copyrights are (for example, the Displacer Beast, which they also think they own, was on the cover of an Alan Dean Foster novel IIRC) but if you want to use those beasties you are running the risk of a lawsuit.

I also recall that some third-party company published a book with nearly identical creatures to the Mind Flayer, DB, Yuan-ti, and several other critters that were removed that was completely OGL, but someone else will have to fill you in on the details there.

Lots of companies have come out with OGL variants of PI DnD monsters. It's really not hard. Most folks who play DnD have access to the core books; if you put one of these knock-offs in an adventure, the savvy DM will know that he can switch in the real thing.