I was trying the new snapshot 1.14 18w43c on a survival map. When I tried to put a diamond horse armor on my horse, it became invisible. It was there, but invisible, when I took off the armor, he became back to normal.
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iron and gold horse armor make the horse invisible too
Killing the horses with /kill @e[type=horse] will cause them to drop the armor and change the tint of the screen to death red.
Please do not mark unreleased versions as affected.
You don't have access to them yet.