Also note that certain creatures have gastroliths now - litterally "throat stones". They're small pebbles used by turkey and, more notably, ostriches, which are kept in the throat/gizzard and used to grind food. While these creatures have no teeth, they still eat things like corn, and the gastroliths help to mill the food by rubbing and grinding against one another. When you butcher certain creatures, you can find gastroliths, made of any stone material and done with a particular gem cut.
[EXTRA_BUTCHER_OBJECT:BY_CATEGORY:GIZZARD]
[EBO_ITEM:SMALLGEM:NONE:ANY_HARD_STONE]
[EBO_SHAPE:GIZZARD_STONE]
EBO - Extra Butcher Object. Associated with the "gizzard" body part, butchering will return an amount of stones with the GIZZARD_STONE type of gem cut - presumably the only source of this cut is from butchering. An actual gastrolith would look like a stone that had the outermost edges sandblasted smooth, with any deep indents or pits left untouched. Unlike regular water-smoothed stones, a gizzard stone wouldn't have any flowing water or tiny particles to sweep into the depressions, so smoothing would primarily occur on the raised faces. They also often show hairline scratches, when a new stone was introduced and cut against the older stones, leaving a small mark before it was worn down to smoothness.
Presumably, you can also add EXTRA_BUTCHER_OBJECT to produce... unique, items. Utilizing the caste system to make distinctions, you can make it where some alligators can be butchered and you might find a toy boat or instrument in the stomach, to simulate that they had eaten the item while still alive. Could also make it so that certain animals produced items on butchering, perhaps have a
giant worm creature that drops a few bolts or swords when butchered. It's hard to tell in that picture, but the monsters from Mabinogi have a series of old weapons lodged in their backs. Or you could have a sort of
Jewel Crab creature, with a gem EBO associated with the shell, causing you to butcher a jewel crab and have it drop emeralds. It's also possible to make Ents that can be butchered to produce logs.
Mostly assumptions on my part, but it appears that EXTRA_BUTCHER_OBJECT simply produces any given item when the creature is butchered, allowing for a vast number of options, essentially letting any creature be butchered to produce any item.
Of note: Eggs wouldn't work. They wouldn't be fertilized nor would they be in a nest box, so you couldn't, say, butcher a zerg queen and get usable eggs - they'd just be edible eggs but would never hatch.