Fun fact: Hardened leather was a component of armor used by vikings, slavs, mongols and later byzantines, and later their varangian mercenaries, and later German empire (pickelhaubes). But it was never a complete set of armor, the hardened leather was usually used as a structural element to something else. Usually steel plates.

Picrel, horses ass armor - For example if you put a sheet of leather on a horse, and let it get wet in the rain, and let it dry.... it will become very hard. You can seal it against future moisture with a glue or oil and it will be moulded armor for that horse, resistant to impact. Or a brace so it can carry more.