Still going monster-by-monster, in order alphabetical. Finally out of the D's...
Let's compare the giant eagle to the roc:
Giant eagles are in the Monster Manual (with their "limited form of telepathy" and their friendship with "certain dwarves and elves") because they appear at key points in J.R.R Tolkien stories and conveniently solve logistical problems that everybody else has been trying to solve for the entire length of the story up until that point.
The roc likely appears in the Monster Manual because of a story in the 1001 Arabian Nights where a guy finds himself at the bottom of a canyon full of gems and suddenly sees the corpse of a skinned goat or cow drop out of the sky onto the pile of coins from above because somebody up at the top of this inaccessible valley thought it'd be a good idea to throw skinned livestock carcasses into the valley so the gems would stick to the carcass because of the blood, and then the roc would swoop down pick up the gold encrusted animal so it could eat it, and the entrepreneurs could then sneak into the the roc's nest and kill it and take all the jewels.
So I ask you: which one of these stories sounds like good D&D to you? Tolkien was an incorrigible railroader.
The ear seeker goes into your ear, lays eggs in your brain, and then--90% of the time--kills you. Up there with the cerebral parasite in the I-can't-possibly-imagine-how-this-could-be-fun category, the ear seeker was allegedly designed by Gary to prevent PCs from listening at doors. It's difficult for me to understand how the mind that gave us the beholder and Demorgorgon and the Eye of Vecna couldn't have thought of a million better ways to solve that particular dungeon design problem.
Eels alone are pretty boring, but you can't really match the eel's face for stupid malevolence. I propose the eel as an excellent constituent part for aquatic bad guys. Right now I'm thinking about a marine medusa with eels for hair and a mermaid body.
Mandy tells me her dad's eels used to escape the tank and slither around the basement until someone found them behind the couch or whatever and then dropped them back in the tank or flushed their dead bodies down the toilet. Which somehow seems more disturbing than a snake escaping. What does the eel want?
Top -heavy. The City of Brass is extremely evocative however. A molten-coloured pitiless 1001 Nights/Sinbadish type hell. I'm not sending anybody there until I can think of some better inhabitants for it. I'm not afraid of anything that has air for feet.
The elements aren't scary. Oh no some water! What the elements can actually do: storms, wildfires etc. can be scary but we're so used to looking at-, and using-, water and fire and rocks that they by themselves evoke almost nothing. I much prefer it when elementals are represented as being some sort of creature. The salamander for fire, the water weird--the earth elemental can be cool if you imagine it as being enough like a hunched, brooding version of The Thing.
The air elemental is very tough to do, since air is really unscary. Nowhere near as scary as it should be. This concept is well demonstrated here.
"A tornado can drive a three-inch nail into a tree, alright?"
"So it hammers a nail into a tree, big deal..."
"Well that is a big deal."
"I don't see why it's such a big deal."
"Could you hammer a nail into a tree by just blowing on it?"
"That's what a tornado does. It blows on a nail, and the nail goes into the tree."
"I still don't think it's a big deal."
"What do you mean 'why'?"
"Why is it putting nails in trees? What's the point?
"To mark them."
Maybe a creepy kind of bird of paradise that suffocates you by looking at you.
Oh, I just thought of something--in a museum in Darwin there's a recording--audio only--of a hurricane ripping through the city. It's apparently pretty terrifying. Maybe that's what you want for the air elements--just youtube some shrieks and howling winds.
Elephants by themselves are like whatever but if you're a medieval europerson and you see some people riding up on elephants that must of been some fucked up shit. Like, not only do these people have herds of these giant-skulled hulking grey snake-nosed, tusked monstrosities but they're actually used to them. Wherever they're from must be totally fucked up.
What are elves?
They're aristocratic--they have all the qualities aristocrats are supposed to have--in a good way (high elves) or in a bad way (dark elves)--aside from stupidity. They're also highly attuned to the natural world. Not in a functional first-hand-experience dwarfy way and not in a "I'm cute so even badgers like me" gnomey way or in a "we're just a metaphor for humble farm-folk" halfling way. You get the feeling that a baby elf is born and the birds all add a new note to their songs and the air thickens and the patterns on the leaves change.
The elf suggests that somehow the behaviour of the aristocracy--its obsession with decor, ritual, symbolism, tradition, sophistication, and stratification--is somehow consonant or complementary to nature rather than completely artificial. Like it's somehow natural to be skinny and pale and never work and love shiny things. Elves are people that you can hear and see but not touch or smell. They are the way some people look to other people. But not the way anyone ever feels about themself from inside their own body. Never trust an elf.
It seems to me like that second head doesn't help the ettin much--in terms of figuring shit out, I mean. When it tells you that the ettin has two heads and also that it's stupid that's not surprisng.
Why not? I mean, if the cyclops seems dumb because it has one eye, shouldn't the ettin seem extra smart for having extra eyes? (And why is it that the one eye seems pathetic, but the two heads don't. Like the extra head is not an asset but it's somehow the ettin's own fucking fault, whereas the cyclops' one eye seems like a curse imposed from without.)
There are smart giants. We can imagine the regal wisdom of a cloud giant or a titan. I think it seems to us that anything with two heads inevitably must be confused. And the more heads something has the dumber it seems. Like the three-headed thing in Monty Python is a complete idiot and it's almost impossible to imagine, say, a hundred-headed beast as being anything but a mindless chaos. No wonder people were so scared of democracy.
Eyeballs are awesome. An eye is a sensory organ and implies something doing the sensing. But, when disembodied, the intelligence it obviously represents instantly becomes mysterious and creepy. The problem here is Gary's already outdone himself. The floating eye is a pretty cool monster but the beholder is way cooler.
EDIT: Rick Marshall just pointed out that the Floating Eye is a Dave Arneson creation, Thanks--please keep doing that, people, I hate being wrong. Anyway, the question now is which came first--floating eye or beholder?
Eye of the Deep
On the other hand, I submit that the eye of the deep is almost as cool as the beholder. Imagine going through a whole campaign and then finding out at the end that the archvillain all along was a bitter little lobster-handed eye guy dwelling deep beneath the waves murkily crafting your demise. The only problem is you can't use both. You either use the eye of the deep or the beholder and that's that.
Unless...they're like a matched set. A cast off pair of eyes from some much bigger divine being. One fell into the sea, one fell into the earth. Both grew bitter and evil, and now they plot against each other.
It occurs to me, contemplating the disturbingness of the disembodied eye, that the Death Star was maybe an eye and the thing in the middle of a TIE fighter, too, and definitely that mandibled mouth in the center of Unicron in the Transformers thing was also both a mouth and an eye.
Eye of Fear and Flame
Skeletons and skulls are always in danger of getting you into Essential Archetype Overload. The way I deal with it is imagining a sort of organized skeletal hierarchy. When the Triumph of Death comes your garden variety skeleton will be the footsoldier, it will be lead by death knights on horseback, and presided over by a lich. And the eyes of fear and flame will whisper tonguelessly into the dull bone curve where the lich's ear used to be.
The eye killer is either a batsnake or an owlsnake--depending on who you ask--from Native American mythology, although it's tough to say there's anything wrong with it there's about a billion other snakey monsters I'd be tempted to use before I got to the eye killer.
If I was running a Middle Eastern campaign I might rethink it. It seems like the kind of thing that might be laying coiled in the bottom of a stoppered urn deep beneath the City of Brass.