So, not really the exactly correct place to put this, since it is modding, but I've just realized how powerful this is.
EXTRA_BUTCHER_OBJECT is a new tag associated mainly with ostriches.
[EXTRA_BUTCHER_OBJECT:BY_CATEGORY:GIZZARD]
[EBO_ITEM:SMALLGEM:NONE:ANY_HARD_STONE]
[EBO_SHAPE:GIZZARD_STONE]
This specifies, in order:
[There's an extra object to be given at butcher:It comes from a category:It comes from everything GIZZARD]
[The item:It's a gemstone:no subtype:it can be any hard stone] - (this means it grabs any known stone, theoretically even gold or maybe even slade)
[Give the item a specific shape:It should use the GIZZARD_STONE type of gem cut] - (likely the only actual way to obtain a gizzard cut stone is from butchering)
Sure, that's fun. Gastroliths! Stones held in the gizzard/throat of many birds, because birds don' have teeth to grind their food but they use pebbles in their gizzards to grind it before being digested. But y'know what's more fun? The sheer amount of options available.
[EXTRA_BUTCHER_OBJECT:BY_CATEGORY:CHITIN]
[EBO_ITEM:SHIELD:ITEM_SHIELD_SHIELD:CREATURE_MAT] - (I don't think "CREATURE_MAT" in specific would work here, but you get the idea.)
This would be applied to, say, a giant crab or giant cave spider. Then, when one of these was butchered, you'd get meat, prepared lung, sweetbread, and a "giant crab chitin shield" that would be instantly usable!
It could also be applied to alligators, simply make the EBO into a toy or an instrument, and associate it with the stomach. You'd catch and kill an alligator, and cut it open to find it had swallowed a steel mini-forge at some point, which you could now smelt for metal or sell off. "Jewel crabs" could be made to drop usable gemstones upon butcher, with different castes of the same creature dropping different gems. Butcher an emerald crab and get a handful of rough emeralds. Butcher a diamond crab and get some diamonds! Butcher a marble boulder crab and get a few marble boulders from your butcher's shop! Add Ents, have them appear on the map's edge and travel slowly with hefty defense, and when you kill them you can butcher them to produce a few logs. You can sleep on the corpse of your enemy! You can embark in a rocky wasteland, kill a few ents, and have enough wood to handle yourself.
Imagine a large worm. About 20 feet long, it's covered in thick armor plates of chitin and moves slowly. Over the years (centuries?) it's been hunted many times, and survived. Old swords, spears, and arrows are embedded in its thick skin, most have been knocked off over the years but many are still wedged deep between the cracks. When you finally kill and butcher the beast, you can retrieve the weapons that are still usable. This could be a new megabeast that drops a few random swords and spears, maybe even add a shield or helmet to the stomach to show how it's eaten its would-be hunters.
Mod in golems, and have them drop a number of stones. Add a more mystical type of dragon, and have it produce a "dragon's heart" type of gemstone, which you could then cut and encrust on a weapon. Change existing dragons to produce spears from the claws - have your butcher attach the claws onto rods and produce dragon claw spears that your soldiers can use. Female giant cave spiders could be butchered for eggs - which could not be hatched but could be eaten or sold. They wouldn't be inside a nest box, and they wouldn't be fertile, so they'd never hatch, but sometimes you kill a spider when it's still got eggs present. Create a "Tumor Soldier" - an old footsoldier that was cursed, skin growing and deforming until he had become a lump of messy flesh with a few bits of armor lodged between the folds, cursed to continue his livelihood of killing but no longer having the sense to care about who he was killing. Butcher them to produce a set of iron armor and weapon.
There's a TON of possibilities here, I've listed several but that's only off the top of my head.
Discuss.
And for everyone who skipped to the bottom of the post, take away the most important part:
Plump Helmet Men can now be butchered to produce edible plump helmets.