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

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.