My first horse was an abuse case. To me it was clear abuse, no questions asked. The horse would go days without being fed, he didn't have a water trough because there was a creek at the bottom of his field but sometimes it would go dry and he would be days without water. He had a hole in his face that oozed puss. It wasn't until years later that I found out it was from an abscessed tooth. They were also physically abusive towards him.
Several neighbors called animal control. Although the horse was skinny and his feet were in need of a trim, he had adequate shelter and a safe paddock so animal control could only educate them on proper nutrition. The owners would do well feeding and caring for the horses for a month or two until the animal control visits stopped and then the neglect would happen again. After many visits from animal control they finally opted to sell.
So, for $500 I got my first horse. I love him dearly, he was three years old and had many behaviorial issues. I've never gotten him past some of the issues, one being that he is extremely disrespectful to my farrier. I have a great farrier who is not only gentle but kind and patient. He does discipline but in a way that never makes me unhappy. It took many years to find someone like him.
My horse is 26 and doesn't have any teeth. He's been on senior feed since way before most horses need it because of that, he's got a terrible wave mouth, a result of the neglect. His feet are terrible and although I don't have lameness issues I do have to spend a lot of time caring for his feet. He's barefoot because he will not be safely shod, that comes from the abuse and the neglect.
Some of his continuing issues came from me not being experienced enough to take on a horse that had been through so much suffering. I'm typing this response to you because I feel it's commendable that you want to help, but it's really important that you do the research. Most of the time a saved horse is costly and takes a ton of work. (A true rescue I mean)
Good luck and keep us updated.