when accessing a horse's inventory and trying to shift click from your hotbar into your inventory it fails to do so, I'm only talking about horses, not donkeys with chests on them, have not tried that yet. But if you have a horse with no chests and such, and try to shift click from your hotbar into your own inventory while riding a horse it does not allow it.
Linked issues
duplicates 1
Comments 2
Duplicate of MC-20718.
I believe this is intentional because of the two equipment slots horses have; one for saddle, one for horse armor. Shift-clicking either of those two types of items will auto equip if the slot is empty. With Donkeys/Mules with chests, shift clicking will move the items from your inventory to the animal's inventory, just like a regular chest.