Let’s get the obvious out of the way. Looking like the guy from The Fly dressed for a Renaissance festival (nice job by Dmitry Burmak in the Bestiary 3), the coloxus demon would make an awesome servant for the Lord of the Flies, Baalzebul—except he’s an archdevil. But if you do a simple alignment swap and tweak the coloxus’s resistances and traits to match the devil subtype, you have a perfectly good denizen of Cocytus ready to go.
That said, let’s examine the coloxus demon the way we should: as a demon. Clearly, these demons of vanity buzz far above the ravening Abyssal hordes. They are polite diplomats while on the clock and refined sybarites in their spare time.
So what does that look like? Because it all sounds very devilish. Still, the demon inside the coloxus is always there, lurking. A devil’s hospitality, once offered, is safe as long as all parties hold to the contract’s many codicils, while a coloxus’s guarantees of safety means nothing the moment any witnesses’ backs are turned. Show one a mirror and the aghast coloxus might attack immediately, witnesses be damned.
I did some thinking, and then the answer came to me: The coloxus demon is Dr. Frank N. Furter. In particular, the temper tantrum, murder, and subsequent dinner party of Eddie (not to mention the numerous seductions/rapes and betrayals and human puppetry).* If you ever want to know how to portray a coloxus demon, “The Time Warp” to “Hot Patootie – Bless My Soul” is all the blueprint you need.
Adventuring in the Abyss, a party comes across a coloxus demon debating society and dinner club. Their refined airs and immaculate tailoring contrast strikingly with their choice of venue—the ever-regenerating ribcage of a comatose titan they are slowly devouring alive. Pointing this out to them—indeed, mentioning their diet at all, or failing to partake—is considered very rude…and grounds for attack.
A coloxus demon seems to have an entire court of aasimars spellbound. Not one seems to see the fly-faced outsider for what he truly is. An adventurer must find a way to stop him, even as the members of court ostracize her for her incivility toward their new guest.
A succubus wants a coloxus demon rival out of the way. She gives adventurers everything they need to know to defeat him. If the adventurers don’t take the job, the succubus makes sure to update them with every new sin the coloxus perpetrates. (The vain coloxus is especially fond of slaying those who were his rivals in life: singers of Cha 16 and above.) If the adventurers do take the job, the succubus engineers the final showdown to take place in a hall of enchanted mirrors, where every glass the scandalized demon shatters threatens to unleash some trapped menace.
—Pathfinder Bestiary 3 72
*If you are the slightest bit tempted to write me an impassioned email about how Frank’s actions are totes chaotic neutral, I want you to know I love you and I respect your opinion. But you are not invited to my birthday party, because you are a crazy person who doesn’t know right from wrong and I’m worried you’ll poison the cake.