I remember one... but suddenly I can't remember a thing about it...
Honestly, though, there are a couple of routes you can play with here.
Route one: bargain with the Fey; as Lea showed during Bianca's ball, the Fey, or at least some of them, are capable of precisely targeted amnesia. So a Fey that trades memories for boons is completely believable. Demons work, too.
Route two: a beastie from the Nevernever that feeds on thought and memories; it doesn't have to be sapient, but could just be something following its instincts; for additional fun, have it be a larvae of something that's alot smarter and is getting older.
Basically, define which approach will work better for your purposes, the sapient bargainer or the instinctual hunter, and then start designing something to fit.