As of 18w03a, the Horse model changed again, including their respective skin and armor files. The skin and armor textures aren't correctly applied to the horse model though, as seen in the screenshots (back texture element located at the horses throat). The texture files seem to be ok, so I guess that's a hardcoded problem.
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A secondary error exists with the rein where the part that goes inside of the horses mouth is using the chest texture instead of the metal texture visible next to the saddle. The magenta portion in the attached image "horse_error_2.png" is where the UV should be, while the lime area is where the UV actually is in game.
I've added an image that shows the texture file and where the UV is being shown. The lime portion of the texture is the horse's body texture, and the magenta section is the neck texture. I've darkened a rectangle between the lime and magenta section to represent where the model is getting the texture from. The actual UV should be moved 7 pixels upward. Should be noted that this is the same for every texture related to the horse/donkey/mule, including armor and markings.