Thanks for the clarification. This is really good to know: If a bestiary entry still shows a "???" drop, that means I still haven't obtained one of the 3 "main drops" from the enemy in question, which is bad news for the enemy, because I need to slaughter more of its kind. Poor Demon, it probably thought its brethren were safe after unexpectedly granting me the "Demon Scythe" and letting me think the "Dark Essence" was its final drop item. ;) Edit: I just obtained a "Demonic Horn" drop from the Demon (and now its bestiary entry shows its three main drops "Mid Potion", "Demon Scythe" and "Demonic Horn"), so I assume my theory holds water.
I think you are correct that the Dark Essence would be the extra drop and the 3 items you listed are the main 3 drops that I gave the Demon! The Demonic Horns can be saved until a later continent and sold, I kind of made them as fossils because Money is kind of hard to get early on and a section on the continent that introduces fossil selling has a 35,000 coin sink in the main quest line, this is the only one I did like this I'm pretty sure.
So I looked into my database and it's actually a 1/250 drop for the Scythe! Wow that is really rare I have killed hundreds and have not seen one haha! But yes the 3 drops you listed are in fact the main 3! So that is a relief.