Hopefully, what else are you going to do with hooves and bones otherwise, throw them away? That's wasteful!

(in reality there's almost no chance, synthetic glues are way easier to manufacture and manage, outside of gelatin animal glues would near-only be used for restoration or specialty applications e.g. some lutherie and other speciality woodworking)

'Terra Preta'

I hope so, horses are terrifying.

You know, it's kind of funny. Horses might have a better argument than any other animal for being the natural predator of humans. Specifically, nomadic depredations on preindustrial societies comprised armies with a proportionally small number of humans and a large number of horses pillaging towns and cities full of horseless humans. In 13th-century Asia, the average horse probably had a better life than the average human. The same thing happened shortly after the horse was (re)introduced to North America in the sixteenth century; settled agricultural societies were quickly overrun by horse-riding nomads.

No, but there are plenty of horse parts in red wine.

Not sure where you're from, but at least for American-made wines that's not often the case, as there are many more common and/or cheaper sources for chitin, casein, gelatin, etc. (pig and cow, as well as egg based sources for example). Also, blood-based fining agents have been outlawed in the US and Europe since the mid 90s thanks to the mad-cow scare.

Of course this conversation also ignores the fact that fining agents are removed from wine before bottling as part of their purpose - they bind with the crap you don't want in your wine. Typical store-bought wine will have no detectable amount of a fining agent in it.

It's contamination "by association." I don't think it's debatable that fewer people would drink wine with pleasure if I told them it was filtered through human bone char, even if I didn't include any in the bottle.

It's more common in white wine.