Because they don't fit the model. Human face (with awesome beard), wings, mammal body.
Besides, we're talking the icons here and every one of the icons mentioned fly in their normal icon anyways.

How about this or this?

well, cockatrice has been shifted to black to start with, but even then, neither of those fit the above model.
If you look at the three creatures I listed out, they all have a commonality to their morphology. Human face, wings, mammal body.

There's also Pixie Queen, which could function as the humanoid race of the plane.

How about a harpy for black?

~SE++

Also, a Chimera could work for green since some could be created with a human face. I mean, they're very little explored in Magic, you could stylize them for green for your plane without really ruining anything.

~SE++

I think the trick with Harpies is that they're not in the same size ranges as the other creatures...

So give them power in numbers instead of single creatures with fat bodies.

~SE++

That sort of defies the idea of an iconic rather than characteristic.
We're looking for these creatures, the icons, to fit the mold.

I was thinking more on some of the interesting tribes of Shadowmoor, specifically the hags. Here's what we've got:

Gwyllion Hedge-Mage
Harvest Gwyllion
Nip Gwyllion
Desecrator Hag
Hag Hedge-Mage
Stalker Hag

It looks like they're a black-alligned tribe that dip into the enemy colors. Since black is, at its core, ambitious, what if hags were dealers? What do they deal in, you may ask? Well, our black-green hags deal in dreams. Or, the sane ones do. Since green favors reality over illusions, the black-green hags hire themselves out as dream-gatherers. During the night they sneak in, undetected (Forestwalk/Swampwalk in action?), perch on the victim's chest and enter the victim's dreamscape. There they rob the victim of his or her dreams and sell them to their employer. Desecrator Hag may have been reduced to a slavering beast from trying to steal the dream of someone or something beyond its skills as a dream-gatherer.

The white-black hags, on the other hand, could work in opposition to the black-green hags. I'm not sure in what capacity, though. Maybe the Harvest Gwyllion's vile epithets could actually be spells of disenchantment or the words to a spell that pierces the hearer's mind to destroy whatever mental guideposts a black-green hag left (to guide them back to a particularly rich area of the dreamscape). It would sound vile due to the defenses the black-green hag would leave near the guidepost; these defenses would make the person who hears the epithet want to get away from the Gwyllion.

Anyway, this doesn't really have a home as I didn't flesh it out beyond my card, the sword, and the idea of an Asian fantasy world where various legendary warriors are all seeking the sword for its power to use for their own ends. I may flesh it out more later, but for now, I guess it can hang out here.

Oh, I got bored and decided to put a little more meat on my Sword Sentai. You can see them here.

Monster world
Werewolves, Merfolk, Vampires, etc etc. All the classic monsters as much as possible.

Not so much gothic horror like Innistrad was, but more B movie base built up as a genesis and full development into its own directions.

So, reviewing some older visual cues from Time Spiral, I ran across this.
And down towards the bottom, we have the visual cue for Vanishing... and it sprang to mind that it would be an interesting character hook for a walker who was displaced and couldn't control the when of his/her travels, who suffered from this vanishing affliction.

The Gates of Galdron
Galdron is a plane of little interest to most, a sort of idyllic haven where nothing seems to happen and nothing really upsets the balance of the world. There is though, perhaps a something wrong with Galdron, because the inhabitants seem to lack some of the verve and urgency of other planes. The peoples of the world live their lives and little happens of any real excitement.
Until, one day, an ancient and massive gate is discovered sequestered away in the wilds. It had been there for hundreds of years, possibly thousands, and virtually no-one had noticed it. Inscribed upon the massive edifice though, was an enormous sigil, the sign of a great flame. The red magi were called to investigate and they discovered that it was an enormous source of power, if only they could tap into it. A number of them gathered together and collectively poured their power into the gate and... something happened. A primal force shifted beneath the skin of the world and the gate opened with a blast of sulfuric air. Behind the door the magi discovered a demi plane, a thriving and vital world composed purely of red mana, of volcanoes and wonders they were unable to dream of... and when the gate opened the other hidden gates scattered across the world were suddenly and amazingly there. Some were even part of cities, but none of the world's inhabitants had ever noticed them.
The gates are linked to each other through five pocket worlds, places of wild adventure and lost treasures... and deadly peril. But as the gates are opened, the walls between the demi-planes are starting to bleed and clash violently, and what that might mean for Galdron is yet to be discovered. Step into adventure, step into The Gates of Galdron!




... I hate being a dynamo of creative energy sometimes.
Incidentally "Artifact Land - Gate"

Altimis just played a Teferi's Curse in my match against him, based on the flavor text, he might have had a little of that problem.

~SE++

Actual Idea: Sunbursts. Sunbursts are sapient, humanoid life forms. They are a pale white, and slightly squishy. They glow faintly from the inside, & the colors differ, but it's never black or white. They have no features on there face, and so they usually draw them on using makeup. Sunburst fashion usually consists of gaudy colors and complicated patterns. There culture is usually money based, putting an emphasis on items.

Idea 2: Cormerine. The Cormerine are also sapient humanoids, but unlike sunbursts, who are not based in any actual color, the Cormerine are a blue based. The Cormerine are said to be drowned creatures. They Look like any creature, except there skin is a pale blue color, and they all have boles ranging from the size of a pea to the size of your head. These boils are filed with a viscous deep indigo liquid. Along with this, they are constantly crying.

You could apply this to any of the Magic races you like:

A race of beings who are united by a sense of self-loathing, rooted in the myths, legends, or truths (who knows, plenty have died over that dispute) race's origins. They are a race of cosmic by-blows that live to break others (both of and not of their race) physically and psychologically. Such acts are what give them a sense of purpose, vindication, or satisfaction. Perhaps they're on a journey into their own psyche to gain a greater knowledge of their place in the grand scheme of Existence. Perhaps they're raging against a hierarchy that they believe forced them to become monsters to have a sense of purpose and belonging. Perhaps they feel more connected to one another in a communal sense by inflicting suffering--a modicum of what they've experienced--on others. Perhaps they simply want to profit off their masochistic bent and accept it as inescapable truth, and thus hire themselves out as thugs, heavies, mercenaries and assassins to the highest bidder.

Hmm, the idea of sapient mana is actually kinda cool. I wonder if they could have a mechanical theme like the Eldrazi spawn that sacrificed for colorless mana....

~SE++

Omnath, Locus of Mana
Just saying, it's been done after a fashion.