I have never seen a setting or a game that really used all the creatures in the MM. They were assumed to be there, somewhere, yes, but did you ever see them in an adventure module? Did you actually seriously use one in a game? Have you seen someone else use one?
If you confirm that you have for any of these creatures and prove me wrong I will hate you forever for the rest of the day.
(Editor's Note: I'm going to take some credit for putting the images together for this one, particularly the ones that took me 9,001 hours in MS Paint. All other images copyright Wizards of the Coast, except for when it is not, derp.)
Achaierai: Okay, so itís a bird... thing. A brightly-coloured bird thing with four legs. It is CR 5, Lawful Evil, from Acheron, can release insanity-inducing gas and like torturing people.
What the hell, Wizards. What the hell.
Animated Objects: Actually this happens a lot but Iíve only ever seen boring stuff like desks and chairs and beds and tables. Why would you animate a table? What benefit is there to taking something intended for the placing of objects upon and allowing it to move around violently?
What about animated cardboard boxes? Animated toasters? Animated butter-churners? Animated backscratchers? Where is my ambulatory shoehorn?
Athach: Abberant giants with an extra arm on their chests. The drawing in the Monster Manual has a pig skin on its shoulder. I wonder what the females look like.
Delver: I donít know about you but I canít think of a use for a lump of rock that eats other rocks and gets wasted off metal.
Actually, yes I can.
Hellcat: Actually Iíve had a DM who put them in. I was a malconvoker and got one to let me ride it. It was fun.
Still, invisible devil-cats. What the fuck?
Digester: This thing looks retarded. Seriously, open your MM up. Page 59. It looks like it just walked off the set of Cthellytubbies. It looks ridiculous. Itís like a dinosaur flumph.
Dragonne: Because dragons and dragon turtles arenít enough, what we clearly need is a cross between a lion and a brass dragon. Where exactly did this come from? Did a brass dragon use polymorph and mount the king of the desert?
Gray Elf: Iíve got sub-race fever! (Sub-race fever!) Itís driving me crazy! (Driving me crazy!) Iíve got sub-race fever! (Sub-race fever!) Iím going out of my mind! (Going out of my mind!)
Ethereal Filcher: I think this thing does get used, but no player wants to admit being robbed by a monoped with a Muppet face on its belly.
Its description doesnít say what that thing on its ďneckĒ is, but since it doesnít say it has two faces, Iím going to assume itís the other end of its digestive system. Yuck.
Frost worm: Basically the purple wormís less cool cousin. If the 4e version turns out to be like the Warcraft one, you know who called it.
Girallon: oh my god is that a gorilla with its head removed and the upper torso of another gorilla stitched on top of it
Gorgon: What the shit is that thing? Itís a metal cow that breathes petrification gas. Save-or-die on a CR 8. Better, I suppose.
Tallfellow: Halflings that are taller and hang around elves.
I make frowny faces at people who complain about elves being ďmary suesĒ and even I think thatís pretty gay.
Hippogriff: Iím sorry, but the WoW version is better. It really is. Probably because it does not look like a horse with a stupid beak. Itís like a girl had a pony and got a little too excited with the feathers and the glue brush at Halloween.
Eleven-Headed Cryohydra: Iím writing this down with the firm belief that anyone who can get this specific with their memory is probably lying.
Invisible Stalker: Iím kind of lost for words on this one. Itís an air elemental you canít see. Yippee.
Krenshar: Exactly how would this species come about anyway? They are described as being ďcatlikeĒ and combining the ďworst features of a wolf and a hyenaĒ, though I donít think either of those species are able to peel its face-skin back to reveal muscle and bone.
The funny thing is Iím sure thereís someone out there who finds that sexy.
Lammasu: Oh my god, itís a winged lion with a manface. It now comes in a celestial half-dragon version!
Locathah: Apparently they were created by Eadro, who also created the merfolk. I like a god with a sense of humour.
Wereboar: Itís not even included on the big picture of lycanthropes they did with the fur-bait werewolf and the werebear that looks like a gorilla in a mask. Poor guy.
Gray Ooze: Youíd think that out of all the oozes, the one discriminated against would be the black one. Nope.
Phantom Fungus: Take the invisible stalker and a fungus and put them together. You now have an invisible fungus that waddles around biting people. What?
Phasm: Itís an amorphous blob that can turn into objects and sometimes leads a life of ďphilosophical contemplationĒ. And the art makes it look like vomit. I hope the artist is aware that he drew a penis.
Aasimar: ďEven now I struggle to type that word without spelling it like buttocks.Ē
Just kidding. Actually I just wanted an excuse to cast weird looks over the Internet at Rob Heinsoo. Ignore this.
Rast: Firey spiders with what look like goblin-rat heads! THEY DRINK BLOOD!
Fuck this. Iím going to paste a statblock for star vampires over its entry. Take that, rast.
Ravid: Dumb floating one-armed snakes that animate chairs for fun. Hurray. (Editor's Note: Further evidence that many MM entries were constructed via Mad Libs.)
Remorhaz: I donít know why, but every time I look at its picture I get the urge to make it into an image macro.
Roper: Ditto. It looks like a tentacle tree stump and whoever drew it gave it an ďOH SHITĒ face, like it just saw a white man tell a particularly hilarious joke about black people.
(Editor's Note: My 2nd Ed. Monstrous Manual has a particularly amusing bit under the Roper's entry: "Habitat: Ropers are not social and rarely cooperate with one another, though a group of them may be found in a good hunting spot. A group of ropers has been named a 'cluster' by scholars with nothing better to do.")
Spider Eater: What the hell is this thing? Itís described as a bat-winged hornet and it doesnít eat spiders. It has Freedom of Movement though. Guess itís a swell mount.
Hellwasp: Show of hands here: Who has actually fought a swam of foul-tempered Lawful Evil bees? That can possess people and corpses by flying up their butt? Well, wasps, but this is close enough and lets me make terrible ďlawful beevilĒ jokes.
Celestial Charger: Or, to describe it fully, the celestial unicorn cleric of Ehlonna. What is this, My Little Pony?
Vargouille: Other people have explained flying demon heads that kiss people to make their ears turn into wings and fly their heads away better than I have. Letís... letís leave it there.
Xill: What the balls? Take me back to the wolves!
Well, I guess evil four-armed dudes who lay eggs in people is at least original and the art doesnít make them look stupider than a particularly dim brick sandwiched between some planks. Letís be nice to them!
Yrthak: Thak yr too.
Itís a pterosaur with a sonic unicorn horn. Itís at the end of the Monster Manual so I donít know if they were phoning this shit in by the end or not.
So, challenge mode: Make an adventure with all of these creatures. All of them. Even the evil butt bees and the inquisitor budgerigars. Do it!