So let me get this straight. They expected you to pay them $100 for putting your horse out in the morning, and you paid it even after you started feeding them and putting them up at night, until THEY finally told YOU you could stop. Now that they are doing nothing and you are still feeding them, you are doing it for FREE.
I don't mean for this to come off rude, but I think an issue bigger than your inability to NOT take care of your neighbors animals because you feel sorry that they would have to watch your horse eat, is that you let them use you as a door mat. You need to get a backbone and jusy politely let them know you will not be feeding their animals for them anymore, unless of course they wouldn't mind compensating you for your trouble.
It's nice that you have such a big heart, but if you continue to allow yourself to be taken advantage of, it will only get worse. That will just make it harder to tell them you're through when you're finally fed up.