Was running through some plains when I found some weird looking mules. Found some horses later on that replicated the bug (also in plains.)
MC-205800
Texture for horses and Mules broken
Resolved
Invalid
0
(Unassigned)
Unconfirmed
glitched_texture, horse, horses, mule, mules, texture
20w46a
Retrieved
i think your texture pack is outdated and does weird things like this... but in any case, this is not question for the minecraft bugtracker...