Before I really begin, I'd like to provide some lovely, lovely stats. By my calculations there are 373 monsters in the AD&D Monster Manual (making it's claim of over 350 monsters on the back cover quite accurate). 202 of those have appeared before, and here are simply updated; 73 have been mentioned before, but are getting stats and a complete entry for the first time; and 98 are brand new.
I've mentioned it before, and I'll reiterate it here. The Monster Manual is not really an AD&D product. It has the banner on the cover, and it's broadly compatible with what comes after, but at heart it's OD&D, a compilation of nearly every monster from all the products that came before it. In many ways it's a beginning and an ending; the last product for OD&D, the first for AD&D, and the first product of TSR's golden age, the five-year period in which much of their most fertile material was published.
Although Gary included just about every creature ever mentioned in a
D&D product, there were some omissions. The most visible ones are
the various entities from Supplement IV: Gods, Demigods & Heroes, but they'll be dealt with in a later book. From the original OD&D boxed set, there are a number of monsters mentioned that didn't make the cut: Sea Monsters
(although you could cover this with giant snakes and dinosaurs),
Cyclops, the Juggernaut, Living Statues, Robots, Androids, Cyborgs (not
quite compatible with baseline D&D), and a whole host of beasties
from the John Carter of Mars books by Edgar Rice Burroughs. The Death
Angel and the Prowler, from later issues of The Dragon, also didn't make the cut.
They weren't created by Gary, or any other TSR staffers, but it's also
possible that they were just created to late for inclusion. Who knows?
In terms of its material contribution to the game, the most important thing introduced by the MM was probably the Devils. They had been mentioned in passing in other products, but they're here in all their glory, ready to incense Fundamentalist Christians everywhere. Other iconic monsters appear for the first time as well: Mimics, Otyughs, Nightmares, Troglodytes, loads of dinosaurs, and many more than I want to list here. But for sheer effect on the game, both mechanically and in the real world, the devils take the prize.
This book also begins the game's slow march to rigid codification. There's not a single monster here that hasn't been altered or clarified, at least a little bit. There are still oversights and omissions, but on the whole the monsters are a lot less ambiguous. Their relationships to the Outer Planes are solidified here as well. We still don't have a detailed description of what the various planes are like (besides what is in that one article in The Dragon), but we do know what monsters live in which planes. The one exception is Hell. There's a lot of info about Hell in the Monster Manual, if you want to piece it together from various monster entries. Allow me to do so:
A BRIEF GUIDE TO HELL
Level 1 - Ruled by Tiamat
Level 2 - Known as Dis. Dispater rules it, from a great iron city also known as Dis. The city is mostly populated by zombies, erinyes, barbed devils, and malebranche devils. Dispater's palace is "infernally grand".
Levels 3 & 4 - Inhabited by Barbed Devils. These levels contain many cells and prisons.
Level 5 - Home to Bone Devils. Ruled by Geryon, from a castle he rarely ventures forth from.
Level 6 - Known as Malebolge. A black stone plane, filled with stinking vapors, smokes, fire pits, and huge caves and caverns. Ruled by Baalzebul.
Level 7 - Much like level 6, but features moated castles that are home to Malebranche devils. Ruled by Baalzebul, who has his castle here.
Level 8 - A frigid level populated by Ice Devils.
Level 9 - Pit Fiends live here, where they serve Asmodeus.
Alignment is also codified here, but it's at a strange half-way point between OD&D and AD&D. Most of the monsters will stick with the alignments given to them here from now on, but the Monster Manual is working with the five-point alignment system introduced in The Dragon. The more well-known nine-point system won't be introduced until later, in the Player's Handbook. For now, Neutral Evil, Lawful Neutral, Neutral Good and Chaotic Neutral don't exist in the game.
Perhaps the most fun I had while writing these entries up (and re-reading them) was in noticing the little tidbits and snippets of information that lie hidden within the monster descriptions. Did you know that a scorpion can sting itself to death? That a skunk's spray can rot magical cloth (and therefore, conceivably damage a bag of holding and cause a dimensional rift?) Or that a Water Elemental can form itself out of ale? It's all super-awesome stuff that I can't wait to use in a real game.
That said, it was a hell of a slog to get through, and I'm glad it's over. Next week I'll start on The Dragon #12, which will afford me a bit more variety than post after post of monsters. As much as I love monsters, it's nice to have a change.