Horses, when equipped with any kind of horse armor, will turn invisible. You can still ride them, and see any name-tags the may have. This glitch affects multiple peoples views.
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Thank you for your report!
However, this issue is a Duplicate of MC-137563.
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Please note, I did find a 1.13 version of this glitch. except that hitting the horse in 1.14 snapshots does not crash the game.