1) It's totally fucked, or
2) Its totally fucked and that's awesome.
Though naturally I'm a lot closer to the second camp, even I will admit that I have no idea what to do with, say, a flind. I don't even know what to do with the word "flind". Jesus that's a terrible word. And gorilla-bear? Hey, Gonzo was my favorite Muppet, but, in the words of this guy:
"Look, either you need a gorilla, or you need a bear. You are never going to need a gorilla bear." (Yes, Jeff, I see you in the back raising your hand. Yes, we know you need a Gorilla Bear. Over at the Armored Gopher. Next to your Spleen Ape and Lava Weasel. Yes. Ok. Anyway...)
(Interlude: I go get takeout sushi for Mandy, then 2 giggling assholes come by on bicycles, one tries to take Mandy's sushi, he fails. I knock him off his bike and punch him a lot and scream extremely loudly and he and his friend are freaked out. They run away. I hate people. Anyway, where was I? Oh, the Fiend Folio, right...)
..Or do I need a gorilla bear? Or something like it. My theory here is every soul can be saved. Every monster soul anyway.
So I'm testing it. My plan is to to find a way to use every single monster in the Folio--to make them useful for me in my game. To accomplish this, I will, if necessary, re-conceptualize and, necessary or otherwise, re-draw every monster in the book. Alright, let's go.
A is for all these guys, click to enlarge them....
So the Aarakocra are like boring intelligent bird-men. They live in the mountains and have flags. I suppose you could make a case for them that they're kinda China Mieville, but China Mieville is kinda Fiend Folio, frankly, and that bird guy in his book was about the least interesting thing in it. Anyway I decided that they were like marsh cranes instead of mountaintop vultures and had sort of numchuk-morningstars and an extra arm/leg and drew that:


Speaking of sucky bird monsters. Really really the next monster was a challenge. The Achaierai is a giant spherical budgie with four super-long legs that "emits" poison gas and is in no way not ridiculous. Seriously look at this:

So two monsters in and I'm thinking I'm thinking the Fiend Folio has completely defeated me. The point is it's a giant bird so tall you can't get to its body and have to attack the legs and I've already used the only bird (crane) that manages to pull of this long-leg-bird concept non-ridiculously.
So I started drawing and praying and came up with this guy:

Next trainwreck: The Adherer. His underwhelming schtick is he looks like a mummy but is actually just a really sticky guy! I can see some late night 50s horror host with a comb-over and a thick Queens accent saying that "Hey kids! It looks like a mummy, but it's actually a real sticky guy! How are Abbot and Costello gonna get out of this one?" This is one of the worst examples of monsters just made entirely in a DM-vacuum by a guy whose players have no sense of wonder or mystery left and just see a mummy and think "Ooh, a mummy, those are hard to turn!" and so is just trying to surprise them. You know the type: Gas-spore, pseudo-undead, ear-seeker, surprisingly-tough-kobold. Meaningless unless you're playing the boringest level-grindiest vanilla D&D.
Plus, the teratological niche is filled: the Lodestone Golem--which I don't know if D&D ever got around to but that Google shows me Magic:TG has, does basically the same thing with less lame. As does the Man of Wounds. Speaking of him, my first draft of reworking the adherer just looked like a dumb Clive-Barkerified version of the Man of Wounds. So I gave that one up.
I decided instead to go with the idea that the Adherer walked on walls and was made of tar.

Now the Aleax: it's sent by your god/DM when you're not following your religion/alignment properly. It looks exactly like you only sparkly. It fights you and has your abilities (and a couple mechanical speedbumps, like it regenerates). If you are defeated by your shiny double you get knocked down to half xp and if you win you get whisked out of the campaign to serve your god for a year and a day. In either case, your campaign sucks.
Instead: let's steal from Thor and Wonder Woman comics and say the Aleax doesn't just show up and try to punch you but actually tries to replace you. It will compete with you in your next epic adventure: it will try to find the Crown of Barskorgenstein first or it will try to defeat the Dread Gorfingel before you do. If it succeeds, bad things happen, if it fails, good things happen.
Since Twilight has ruined "shiny" for at least a decade, we'll say it looks exactly like you except for a certain shiftiness in its eyes. Friends and acquaintances could easily confuse the Aleax for the genuine article. Its conversation is dull, however, and it will only speak and act like a tediously pious and platonic representation of your faith and/or alignment. It takes this "trying to replace you" business seriously and will attempt to supplant and stymie you at every turn.

My first try (an abandoned draft of which turned into the Adherer) was too sci-fi for my D&D game, though I include it here in case anybody wants to use it on Planet Algol...

In my second draft I decided the whole vegetal-mastermind thing was creepier if it wasn't even remotely human. This weird gooey thing just sits on a Faulknerian branch somewhere and makes life miserable for you. I figure it controls trees by means of coating them in its own algae-slime which is a sort of continuous body sheath that places whatever it coats under the algoid's control.

The Al-Mi'Raj. A bunny with a unicorn horn. I figure if I can pull this one off the whole rest of the Fiend Folio is cake. Garbug? No problem! Bunyip? Bring it on!
So I wasted a lot of time googling Watership Down and Svankmajer's Alice and Rats of NIMH and came up with this:

Is it not-dumb yet? At my table I figure I can pull it off--it needs a low raspy voice and it's full of hard-won animal-wisdom and twitchy bitter-little-prey-animal head movements. I think it will work. The black nightingales will speak of it in low whispers: "Consult the Al-Mi'Raj, human..."
Or maybe it's still just a bunny. Viv thinks it's adorable, but her taste is suspect. Test drive soon and I'll let you know.
Next up: Oh joy, another mechanical and lexicographic variation on a ghost--the Apparition. It attacks for no reason and you save twice or die. Surprise! Who thought that up? That's barely a game. And the worst part is the original Russ Nicholson illustration is great. As are all of his skeletal undead. (The apparition is the upper left there). So I've got nothing to work with, really.
Alright. I'd say, psychoetymologically, the apparition is distinguished from the garden-variety ghost by its laziness. The apparition's main thing is it appears. It's main disturbing thing is showing up. This is the intimations-of-psychosis-is-that-guy-really?-there ghost, not the rattling-chains-magic-weapon-to-hit ghost. This is like Hamlet's father. The apparition just shows up, does almost nothing, and makes spooky demands on the living.
Also, you won't know the ghost is a ghost, necessarily. It looks just like a creepy living person. Then a few hours later you go--wait, didn't that guy die already? Ok, honestly this is more a plot device or puzzle-initiator than a monster, but that's what ghosts usually are, so I'm cool with that.
Mandy says I should've drawn him wispier, but I drew wispy things for like 3 years in the Zeros and I'm over it.


Anyway I added this bit since I actually drew the picture here (with a guy's head in the bug's mouth if you look carefully) while GMing the following Call of Cthulhu scenario: the bug is a servant of followers of the Insect God En-Gorath. The device that brings the bug into our world can be any object. The object is typically disassembled or in fragments (a house, a sculpture, a painting, a lamp, etc.). When it's reassembled, the Assassin Bug will crawl out of the thing and slay whoever completed it. So the typical tactic is to use some object that the cult knows the recipient will want to assemble (they will feel a strange compulsion to assemble it, thought the pieces may be widely scattered).
Last and least in the A's is the unbearably dull Astral Searcher. This 2HD-attacks-the-nearest-creature-for-no-reason-and-if-it-kills-you-it-possesses-you bundle of stats is so dull it doesn't even have a picture. The only interesting detail is they're secretly formed by traumatic events or spells being cast on the astral plane.
So naturally what you'd wanna do is tell anybody going to the Astral Plane that mucking around too much up there could cause the formation of an astral searcher, then make a table for that with some percentages and modifiers that you roll on whenever they do anything and make your PCs all scared about it and be all "Holy fuck, if we do that, it might create an astral searcher!" in high-magic or high-drama situations and then be a clever improvisey DM and give each individual searcher powers ironically derived from the trauma that created the beast in the first place. Like: if it was caused by a Chain Lightning spell then the thing has the ability to deaden electrical signals (like nerve impulses, f'rinstance) in some sort of contagious chain-lightning-esque way.
As for the picture, hey, I can do whatever I want. There's an astral searcher:

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Please feel free to tell us what you did with these guys in your game in the comments. One of the best parts of the Alphabetical Monster Thing I did a while back was hearing the readers' take on all these guys.