Ok, so I totally screwed up the list by forgetting to include any undead. Notre that I scribbled that list down in a couple of minutes without referencing the books - something was bound to slip through the cracks. But rather than edit the previous post to make myself look good, I'm going to have another go.
WHAT'S COMING OUT: The Chimera is out, leaving the Manticore as my sole representative of mythical beasts. I've reluctantly pulled the Wyvern as well - dragons fill that niche already. Black Pudding? Gone, since the Gelatinous Cube is way cooler (and more on him tomorrow). And sorry, Mr. Kuo-Toa - I love ya, but I need at least one demon.
ADDITIONS: The list needs some undead. In the interests of covering low, medium, and high levels, I wanted to put in Ghouls, Wraiths and Liches. I find Ghouls a lot more interesting than Zombies and Skeletons, and I needed Wraiths as the obligatory level-draining monster (with a cooler name than the Spectre). At the top it was a toss-up between Liches and Vampires, but Liches are much more of a D&D thing. But I can only fit two things here, so the Wraith is out.
I also need a Lycanthrope. In the interests of including the most D&D of the bunch, I went with the Wererat. It's not as iconic as the Werewolf, but it's another monster that you really only see in D&D and D&D-style fantasy.
Lastly, my token Demon or Devil. First I narrowed it down to a Demon, because I know them a lot better. Most people I know are also more familiar with them - they all know what a Balor or a Marilith or a Vrock is, but Cornugons, Gelugons, etc. they can't tell apart until you show them a picture. I'm going with the Balor, just for being the ultimate demon. If you want to have a bad guy summon one, it might as well be powerful.
So the updated list:
FIRE GIANT (Someone wanted to know why I picked this over the Frost Giant - I think that Fire Giants are a lot more versatile in terms of what terrains they can frequent.)
BALOR (or Type VI Demon if you prefer)
It's certainly a better list than the one I had jotted down previously. But if this experiment has taught me anything, it's that D&D has far too many cool monsters to condense down to a list of twenty.