Some of the spirits in this list didn’t quite fit into one of the other categories, but were too good (so to speak) to be excluded. Others were just amusing. But this is intended to be a writer’s guide to demons, and most of the spirits here suggest a plot, or at least a few plot complications.
A child-stealing demoness once hanged by her hair by Solomon himself
A goetic dragon with a strange relationship with the dead
The 3-headed guardian at the gates of Hell
Canaanite god of grain, fertility, vegetation, storms...but probably not fish
Mukīl rēš lemutti
An ancient example of "demon on the left shoulder, angel on the right"
A mysterious demon whose name means...name
Hell's mimic, the master of dreams and visions
The Promethean angel that gave humanity writing (and thus, doomed them)
Sometimes demon prince, sometimes Angel of Death, a powerful, complex spirit.
Sigambach and Rabidmader
A pair of demonic notary publics, keeping a record of sigils and pacts?
Ancient Mesopotamian spirit that lurks in the bathroom
For someone at the top of the demonic hierarchy there's not a lot out there about this one...
Vine / Royne
Building towers and fulfilling monogamous relationships...also, Hell's official witchfinder.
A modern demon born from urban legend