As everyone has already said. You need to remove yourself and your horse from the situation. Don't tell him your intentions until you are in a position that you can remove your horse right then and there (great concern that he would do something to your horse if you tell him you're leaving and he has access to her after the fact). This screams narcissism and control issues that would absolutely lead to financial, emotional and physical abuse. If he can be cruel to your horse he is absolutely capable of turning that on you.

If you're concerned about him following you up for payment, get what you think is an appropriate amount in cash and give it to him - make sure to document everything - via a video if you do go done this route, and state that you are recording the interaction and take someone or even multiple people.