There are a few texture bugs with Zombie and Skeleton horses, as well as the Mules and Donkeys

More specifically:

In the skeleton horse texture, the saddle is outdated, however, the new texture appears on all other horses and donkeys

Skeleton horses also have an outdated chest texture that appears on all other horses and donkeys except for the skeleton horse

Zombie horses have a pouch instead of a chest in their texture

The ropes of the saddle in the zombie horse texture are slightly different than all other horse and donkey textures

On the mule and donkey textures, the brightest part of the metal on the saddles is inconsistent with other saddles, since the brightest part is just a random color on the text on the fur of said donkey or mule.