For context (and vague spoilers) -- I already know the secrets of the... rats, gargoyles, minotaurs, guardians, purple slimes, lovers, mine king, rat king, beholder, purple mage, chests, gnome (maybe). I hope there's something to discover about skeletons, bats, or green slimes, but so far they seem completely random to me (identical except for their power level, before the slime scroll).