Occurs both on creative and survival, I opened a LAN world and my dad could not mount a horse, whenever he tried it reared up. Eventually he did get on but as soon as he did he lost connection and could not return to the server.
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Duplicate of MC-13628, please use the search function to see if your bug has already been submitted. Currently over 52% of tickets are being closed as duplicate.
You also can't mount a minecart and pig :'(