The horse textures all contain the chest bag texture, despite the fact that horses cannot have chests equipped onto them currently. Donkeys and mule however should keep this texture, because they use it.
Effectively, they are unused pixels which mislead resource pack artists.

Unused Pixels:

Take as an example, horse_black.png:
(This is not the only affected texture, it's just to avoid redundancy)

Affected textures:

• horse_black.png
• horse_brown.png
• horse_chestnut.png
• horse_creamy.png
• horse_darkbrown.png
• horse_gray.png
• horse_skeleton.png
• horse_white.png
• horse_zombie.png

Notes:

1. This is an extension of MC-272100 and very closely relates to it, but for the chest specifically and not the random palette pixels.
2. The donkey and mule both use their own textures.