[spoiler] Critter Drops.
So, I know that at least one person has found a Drake Scale. This leads me to believe (hopefully) that others have as well.
Where did you find them? :)
I've been trying to collect data on what critters drop what stuff and at what depths.
Time to start sharing the knowledge?
I'm still not certain on how to properly organize the information, but thus far I've been going with entries containing:
Item, CritterName, Depth.
And when the same critter drops the item at a lower depth I replace the depth in the listing. Thus you know the critter type the item that they can drop and the earliest depth they can drop it at.
My logic is that some maps are redundant at different depths and even if the critter may change a bit in display (like the yellow wolver vs. grey wolver) if it spawns in the same spot then it is the same base critter. I've yet to run into a situation where something like a chameleon is replaced by a jelly. This does assume that the different iterations of critters share the same drops.
Here are a few datapoints I've found regarding some of the more useful stuff.
Blasting Powder, Goblin Demo, D14
Blaze Pepper, Wolver, D11
Brute Core, Cube, D11
Bushy Tail, Wolver, D20
Cooling Cell, Gun Puppy, D7
Drake Scale, Bull Chromalisk, D25
Dynamo Force, Tree Golem, D20
Fine Fabric, Gremlin Worker, D11
Fine Fabric, Gremlin Soldier, D20
Fine Fabric, Slime, D11
Fine Fabric, Blue Spookat, D7
Fine Fabric, Spookat D11
Force Dynamo, Redward, D19
Fuel Canister, Gremlin Pyro, D20
Fuel Canister, Chest, D12
Heavy Gear, Redward, D19
Plasma Cell, Gun Puppy D22
Power Cell, Gun Puppy, D9
Primal Ore, Slime, D19
Reaper Rib, Zombie, D18
Rocky Core, Cube, D19
Royal Core, Cube, D19
Shadow Steel, Devilite, D16
Silver Coil, Gun Puppy, D22
Silver Coil, Goblin (Melee), D20
Soul Dust, Skeleton, D11
Soul Dust, Spookat, D12
Sulfur, Deviling, D10
Unstable Core, Blast Cube, D22
Warp Dust, Wolver, D9
Zee Core, Sleepy Lichen, D15.
Please share your knowledge. If you've seen an item listed above drop from the critter at a lower depth please mention it :) If you think there's an item that should be listed please post about it with any datapoint you might have.
Fine Fabric, Gremlin Worker, D11
Unstable Core, Blast Cube, D22
Warp Dust, Wolver, D9
Soul Dust, Skeleton, D11
Brute Core, Cube, D11
Soul Dust, Spookat, D12

Cooling Cell, Gun Puppy, D9.
Zee Core, Sleepy Lichen, D15.
If you want to contribute to the wiki, please add all critter drops to the appropriate monster page! :)
Drake Scale, Bull Chromalisk, D25
Emerald Rook - Concrete Jungle - Blight Boulevard I or II, I can't remember.
Cooling Cell - Gun Puppy D7
Fine Fabric - Spookat D11
Soul Dust - Zombie D11
Power Cell - Gun Puppy D11
Edit: Miracloth - Spookat D22
Plague Esence - Poison Zombie D24
Zombies/Droul - Reaper Rib, D18+
Goblin demo - Blast powder D14+
Devilites - Shadow Steel D16+
Bushy Tail, Gray Wolver, D20 (Wolver's Den)
Reaper rib - treasure box (behind energy gate) Level 27
Blaze pepper - ash wolver - Level 11
Force Dynamo/Heavy Gear - Redward - Level 19
Plasma Cell, Gun Puppy, D22
What is this I don't even
Mug of Misery, Devilite Pit Boss, D15-17ish, can't recall
I hadn't seen this item before, says it's 4-stars, anyone know what it's good for, if it's good for anything yet?
Miracloth from Silkwing Butterfly D19

@Mesona: Updated the Wiki. :D
@airplane: Weird. Do you know if Mug of Misery is a crafting item or something?
It's a new item. I found one as well somewhere on floors 19~21, and currently I do not know of any recipes that use it.
Blaze Pepper from Darkfang Scorcher on D19

Found 3x Forbidden Fruit from basic devilings
between Haven and Moorcroft
2 stars
uses the "core" icon
"A sinful snack favored by famished fiends."

Found Phial of Phear dropped from a Devilite Overtimer.
Around D16.
@ saphykun
It's definitely a crafting item, it has the same style icon as a cooling or fuel cell, only purple.
I got that miracloth on a level between Moorcroft Manor and Emberlight. Unfortunately, I can't remember where or how. :(
I'll try to keep a better eye on where I get rarities and let you know what I find.
And so:
Power Cell, Gun Puppy, D9
Sulfur, Devilite (Worker and Overtimer), D10
Fuel Canister, Chest, D12