I found that if there is a wall behind the nether portal in the dimension you are trying to get the horse to travel to, the horse won't go through as the horse is 2 blocks fat so needs space around the portal to go through.
Sasha Temple
I found that if there is a wall behind the nether portal in the dimension you are trying to get the horse to travel to, the horse won't go through as the horse is 2 blocks fat so needs space around the portal to go through.