Some horses spawned in the ocean, so we decided to use boats to take them to land. When getting to land and trying to destroy the boat, the hit box for the boat was inside the horse, meaning we hurt the horses every time we try destroying the boat. Trying to ride the boat will make you ride the horse instead.

An easy fix for this is either to make the hit box for the boat bigger, or the hit box for the horse smaller.

EDIT:
I see now that I duplicated another bug report: https://bugs.mojang.com/browse/MC-89963