My very first horse was a 17hh chestnut mare that my aunt bought from a horse trader. She was thin, malnourished, and seemed like such a sweet heart. The vet came out, vaccinated her, did blood work, we wormed her, had the farrier out, etc. We got her all UTD on the TLC she needed. Once she started to put on weight and "feel her oats"- she seemed to go "sour" . She would rear, buck, you name it. We ended up just using her as a broodmare for 1 last time. She retired and passed away as a pasture ornament.
My advice is, don't be fooled by how they are acting when you first meet them! Somewhere in there is a beast who knows how to move! I've always been told that they have no sense of direction. They know how to run forward and that is all. Re-training may be complicated.