Allow me to propose a heretical thought: Dungeons & Dragons has too many monsters . I think, deep down, we all know this is true, but let's consider it more carefully.
The first AD&D book I bought, shortly after Christmas 1979, was the Monster Manual. I ordered it from the Sears catalog, if you can believe it. Talk about a different world. And, of course, I loved the book, spending hours and hours reading and re-reading its entries and poring over its illustrations (and not just the succubus). In many ways, it was probably the singlemost important D&D book I ever owned.
Yet, if I were to go through the book systematically and check off every monster within its 112 pages that I'd ever used in an adventure, I conservatively estimate that it'd be no more than half of them. If I expanded my search to include both the Fiend Folio and Monster Manual II, I suspect that percentage would drop well below 50%. Why is this?
Part of it is that, even in my younger days, my conception of the D&D "world" was always a measured one. That is, I strove for some degree of plausibility and that meant I had to reject the inclusion of certain monsters, either because they were simply too outlandish for me or because I already had more than enough monsters in a given ecological niche.
As you can see, Gygaxian naturalism isn't something I just made up for a blog post, but something I intuited from reading D&D itself, including the Monster Manual. In the breadth and depth of its entries, the Monster Manual gave me the tools to be able to create a coherent world, at least within the bounds of the shared fantasy of the game. I had a full array of predators, prey, and scavengers to build multiple fantastic ecologies and indeed did so.
Now, one one level, that's a wonderful thing, because it meant the Monster Manual was a true "toolkit" book, providng referees with a wide array of tools for almost any job, including many jobs they'll never undertake. Had the Monster Manual been the only monster book for AD&D, I probably wouldn't have proposed my heretical thought. The problem only really becomes clear once you start adding more and more monster books to the game, the end result of which is a rising tide of incoherence, at least if you try and find a place for all the monsters in these books.
More than that, though, I think a smaller selection of monsters provides a stronger foundation for building a fantasy setting. Including too many monsters is a great temptation, even for experienced referees; doing so runs the risk of overwhelming the "groundedness" that I feel is vital to a good fantasy. Likewise, monsters, like magic items, lose much of their appeal when they become merely entries in a catalog rather than occasions for wonder. As D&D acquired ever more monsters in hardcover format, this became a problem I encountered often, both in my own campaigns and in those of my friends and acquaintances.
For myself, I must admit to preferring the smaller monster lists of the three little brown books, Holmes, and Moldvay/Cook. Their listings are broad enough to give a lot of variety and deep enough to challenge characters of many levels of experience. To these lists can be added individual "specialty monsters" on an adventure-by-adventure basis rather than in the more magisterial approach favored by the AD&D hardbacks. I personally think this approach is more easily manageable, leads to greater coherence, and contributes to that sense of wonder that the best fantasy engenders in us.
Lest there be any misunderstandings, I am most emphatically not saying I hate monsters, monster books, or the original Monster Manual. Judging from past experience, some people take what I say in my musings to be dogmatic statements when they are in fact tentative thoughts expressing my current (often incomplete) feelings on a given subject. In this case, my feeling is simply that, far from losing anything, D&D would probably gain a great deal in terms of focus, consistency, and fidelity to its pulp fantasy literary forebears, if it pared down the list of its monsters to something much smaller than the bloated mess it acquired over the years.
Am I wrong to think this?