Well, I'd say Demogorgon looks pretty freaky.
The thing with any kind of planar epitome, I'd say: there's more than surface appearances to consider. There's the potential allegorical deapths which the physical form represents. The natural shell is merely that: a shell. It often depitcs most accurately what is in the mind and attitude of that which it houses. This is especially true for planar paragons (demon princes, archdevils, archangels, eladrin courtiers, modron heirarchs, formian queens, slaad lords, guardinal pridelords, etc). These are creatures made of belief...their own belief basically takes on a physical form when they conceptualize themselves.
So you can't just say: "Would I be scared of this?" You have to ask yourself: "Am I terrified by what this represents?" anything with a Polymorph can look scary in a dark alley. It takes a true planar paragon to frighten you with what they *are.*
So, ponder what Demogorgon represents, not just what he looks like...
So, think of Demogorgon's twisted, twitching body...sinuous, secretive, topped with horrifying visages of mad beasts. Think of what a hyena or even a baboon may represent: the madly joyous ripping and tearing of flesh, a savage predator so jovial and happy, flamboyant in color or loud in laughter, as thirsty for pain as it is joyful in inflicting it. Sadistic, mad, uncontrolled even by itself, more instinctive than human, more bestial than rational.
Ponder the snakiness of it, the sleek agility and unparalleled flexibility of this icon of chaos and wickedness. Every part of his body grasping and curling and entwining everything, killing what it touches by sapping the life from it. They don't smoothly wrap or entwine, either. It's not an easy motion, but a jerky twitch, an almost nervous panic...you can feel the nearly erotic tension in the creature as it savors the sensation by twitching and tingling...it's almost like he's a cold hand sinking into hot water...it hurts and feels so good all at once to him. It causes his ichor to boil...he wants to savor the sensation. If he could sweat, he would. It's a fix...a glorious revel in the feeling of sapping the life from something. Even when not entwining a victim, his body twitches and convulses with nervous energy, ready to satiate his addiction to the lives of others.
Think of the dual nature...the pulling and pushing of each other, the intense hatred for the two bound together, each wanting to do their own thing, each craving the obliteration of those who threaten it. Think of it barely holding back it's own belligerence to itself....it's tail constantly whips at a head while a tentacle or two comes just short of slapping itself. The two pull apart when they can, hitting different targets, dividing their attention, pretending the other isn't around except when they must unite. The insantiy of it. Think of being caught by your shirt or by your foot just as you want most to run away -- both heads experience this same terror and hatred of each other at all times, each in the middle of running away from the other.
Demogorgon currently rules the Brine Flats (or the Gaping Maw), a realm of salt water and rocky prominences. The Styx flows into it via a whirlpool. Demorgorgon's monstrous servants commonly include krakens, aboleth, hezrous, aboleths, scrags, and skum. He is a master of wicked constructs as well. The Ixitxachtitl worshiping him wouldn't be too out of line....though they are said to revere Erythnul above all according to the MMII.
It's this humble gamer's opinion that the Brine Flats are not Demogorgon's original home, but merely currently where his power base lies. Demogorgon is a mental character, a dark twitch at the back of your mind, an impulse to hurt oneself or others barely controlled, but which would give such glee if one gives in. IMHO, paladins who take up the persuit of evil a bit *too* cruelly are in constant danger from the lure of this creature, this gleeful bloodthirst...
I think Demogorgon is definately more than just an aquatic evil...what, I don't know...but, after all, Orcus wasn't always a ruler of undead...
It may be an interesting inverted trinity of sorts: Demogorgon (spirit), Graz'zt (flesh), and Orcus (ruler), but that may be giving too much credit for order to a parody for the Abyss. Just as likely, they were just the toughest of the toughest...Graz'zt probably has a few billion secrets about what he was, too...