The bug
Skeleton Horse:
The Skeleton Horse saddle still uses the old saddle texture from before 1.14 (the Texture Update)
The Skeleton Horse's unused chest also uses the old chest texture from before this update as well
Zombie Horse:
The Zombie Horse's unused chest still uses the DrZhark pouch from Mo' Creatures instead of the Vanilla chest
Donkey/Mule
Bits of the Donkey and Mule's saddles have stray pixels on them that seem to match up with the Donkey and Mule's fur colors respectively, gray on the Donkey and brown on the Mule, clearly indicating a mistake (fur/flesh wouldn't be on a saddle texture when none of the other horses have it)
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Can confirm for 1.18. pre-1
It seems for the donkey and mule the colors on the metal part of their saddle come from misplaced fur, as it matches up with some pixels on them.
Zombie horse modded chests are never visible ingame
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