I think that in general devils should look more consistent and alike than demons do. That imp back in the other post, or the abishai from way back in the first post of the series? Those are my template for demonkind. Here they are again if you want a refresher.
So keep that in mind.
I actually have a really specific vision of what a barbed devil should look like in my head, and it comes down to this: Porcupine quills. This should be a devil with lots of porcupine quills. The illustrations are always this weird chupacabra guy and I hate 'im. Sadly I couldn't find an exact illustration of what I've always thought they should look like. Here's a couple of devils that do have nice quills though.
A lot of the old illustrations of bearded devils look...grubby. I don't like that, I think a devil should tend to look professional, I guess. This 4e Dragon Magazine cover does something interesting by making the beard out of snakes. That could work for me. Though actually... I also kind of dig the idea of the beard being a bunch of horn spikes, or having blades braided into it or something. I dunno.
UPDATE 9/16: Had a thought yesterday I quite like: The beard has burning strands of matchcord or embers in it like Blackbeard, wreathing the face in nasty, brimstoney smoke.
Actually, not too bad either, though some old versions of them have insect wings that I thinktotally take the wind out of their sails. I'd like to see more typical horns and wings on one of these just to see how it goes. I think the name for them is kind of bad, something like "hunger devil" or something might have been cooler. Or else give them some better connection to bones than just being skeletally thin.
My brother is really fond of these guys. I think they look like knockoff cenobites. But the idea of a "chain devil" works for me, I could see cool stuff happening with it. I hear pathfinder spun Kytons off into their own thing separate from devils. I don't know if that's an improvement or a detriment to them. I see them as having a couple of great big heavy piercings (one on each arm, one between the wings, one between the horns, et cetera) that they hang chains off of. Sometimes they're bound by nailing the chain to one. Maybe a bit like this?
Fallen angels are good. I like them more as punishers, as their name implies, than as the lawful equivalent of succubi. I think Pathfinder might have wrote them that way? They drag you to hell. They probably get a lot less pretty as they get more pissed. I don't think they're actual devils, but they are part of the forces of hell.
Well... aren't they all? I like horned devils to have big, impressive ones, maybe unique. Antlers are a good look, but also those times Hellboy's horns have grown out all the way are good. Or maybe they have rhino or triceratops or styracosaurus horns? Lots of good possibilities here. I tend to see them as being more on the aristocratic, lordly side, but powerfully built.
Actually, this pic was a factor in me thinking about the appearances of demons and devils in the first place. For some reason this picture totally says "ice devil" to me. I've never been huge on the insectoid appearance of the ice devil traditionally, because it seems so inconsistent with the rest. A beetle carapace on thisguy could be kind of interesting.
Aren't lemures just damned souls? I dunno, they probably just look kind of ghosty. I can't remember the last time I thought too hard about it. Properly speaking I don't think they count as devils. Maybe they're all chained up like Jacob Marley?
These guys have a problem: They're just lawful balors. Solution, make them more princely, more Mephisto. They are men of wealth and taste. Probably wear robes.