The simple answer is this:
You can't use ANYTHING in WotC books.
Exception: The final two creatures in the MM2 (originally from the CC).
That is the ONLY exception. Why? Because that is the only thing WotC has EVER published (in hard copy form) under the OGL.
You cannot use stuff from the Players Handbook. You cannot use stuff from the Dungeon Master's Guide. You cannot use stuff from the Monster Manual. You cannot use stuff from Deities and Demigods. And so on... if it is a book published by WotC, you cannot use ANY of it.
What CAN you use? The System Reference Document. You alluded to it at
http://www.opengamingfoundation.org/srd.html - though I don't know if it has been updated yet (check the link to the WotC site).
In other words, if you want OGC, you must forget that the Player's Handbook, DMG, MM, MM2, and whatnot even exist. Go straight to the System Reference Document. Do not pass go. Do not collect $200.
So you can't use "Hook Horror" (though I suppose you could reference "two of the beasts illustrated on page 34 of the MM2") since they can't exclude you from mentioning "MM2" ("Monster Manual 2" is different).
Dumb? Perhaps. But that's the way it works.
You are correct in your final question. If it's outside the OGL, you can't touch it. All of WotC's books - except as mentioned specifically above (2 creatures) are outside the OGL. (They can do this because it's their system, though I really wish they wouldn't). It's as simple as that.
--The Sigil