I can't recall, but do the books mention redcaps?
They're nasty buggers. They can change their size from wee little fellows to hulking brutes. They get their name because they kill folks and dip their caps in their blood, staining them red.
In the Dresden-verse I imagine that they'd end up being hitmen and thugs.
Mechanically, redcaps probably have some limited shape changing. The little form has Diminutive Size, while the large size most likely has Inhuman Strength, Toughness, and Recovery. Most likely has the standard iron Catch.
Sluagh are pretty cool. They're thought to be the spirits of the dead not good enough for Heaven, but unwanted in Hell. They fly through the night like flocks of birds, and have been known to swoop up travelers at night and carry them off into the night. They're also thought to sneak into the homes of those on their death bed, hoping to carry off the person's soul to join their masses. On rare occasions, they'll rescue people from shipwrecks, but that's thought to be really bad luck for the saved.
In the Dresden-verse, we don't tend to have actual spirits floating about all wily nilly. So perhaps they're a type of faerie that crafts more of their own from ghosts before they can go off on their own or fade.
Mechanically, probably has Wings (that likely look like some sort of clothing when not in use) and Cloak of Shadows. May optionally have Glamour or Unseelie Magic.
