Thanks for the encouragement.

Please add any ideas that you might have or that you've already used!
On to the D's of the Monster Manual:
Darkmantle: Svirfneblin outcasts are subjected to a ritual that causes skin to grow over their eyes, and cast away. A ruthless society of these has adapted, causing flabby wings to grow between their arms and legs, and echolocation slowly becomes used. Their darkness becomes a sorcerous ability, and they cling to cavern ceilings in groups, causing darkness, then latching onto the heads of prey with their flabby bodies while biting them with their sharpened teeth. (Constrict damage results from bites, suffocation, neck-wringing, and constriction of their flabby wings.)
Delver: Drow (or any race, really) terminated by drow society (or another evil race) are sometimes scarred with acid and thrown into large pits, where an appointed hermit will chant over the pile of corpses for a fortnight. This draws the spirit of the cavern into the pile of corpses, which then animates. (Story: This can be done as a last resort to protect a home, or as sort of an intimidating dog guard -- "the soul of our land protects us through the bodies of your slain.") (Another sketchy almost-undead, YMMW, but the spirit is alive, simply incarnate in corpses.)
Demon-Babau: Archon scouts, protective aura of holiness instead of acid. These sometimes serve as forward scouts for angelic armies. 2 Claws are two shortswords, bite is an unarmed attack.
Demon-Balor: Again, negate the alignment, give these stats to a towering flaming angel with skin of platinum, and you have a good Avenging Angel, called forth to punish the wicked. Powers become good instead of evil, just drop insanity, and perhaps change the fire powers to electrical in nature if you want to disguise the stats more.
Demon-Bebilith: A giant that ate a cursed spider awoke one day to find his lower body had transformed into that of a terrible spider, and that he was able to throw webbing and spit venom. Claw attacks are instead club attacks. Sort of a giant-drider hybrid. Plane shift doesn't really fit the concept well -- perhaps the curse only allows the giant to teleport to a plane where he is punished by spider-spirits for a time, before being allowed to return.
Demon-Dretch: A specific curse turns humans from a small village who venture too close to ruins into long-armed, clawed, tusked versions of themselves. (Summoned dretch are humans from within the cursed ruins, who will reveal the nature of their curse if questioned by their summoner.) Those affected by the curse sometimes speak in hushed, sickly (abyssal) tones, and seem to know what each other are thinking. (Curse idea is that a dwarven lord's tomb is underneath the town, and does not appreciate the human filth atop him.)
Yikes, there are too many demons... hah.
Again, thanks for the encouragement, I hope some of these are useful, and post any ideas you have of your own!