If you switch resource packs and then take armor off a horse the horse will still appear with the armor on. This is how to duplicate:
Use any resource pack (I used Faithful 32x32)
using a spawn egg spawn a horse
tame the horse and saddle it
put armor on the horse
switch to the default resource pack
take the armor off the horse
observe the armor texture still on the horse
This is a duplicate of MC-18813 but it was closed and I didn't think a comment would be seen.
Linked issues
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Duplicate of MC-16170.