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.