Ok. This account suggests you are a child being put in a dangerous and impossible situation by an adult who is irresponsible and made bad decisions. Most of the time partners make good enough decisions. But I have certainly seen parents make equally stupid decisions about horses.
You don’t need to tell us but you may want to reflect on whether these poor decisions are part and parcel of how your mother behaved (impulsive, fantasy based etc).
Your mother acquired a feral horse that is beyond her skill set and she gave up working with her and suggests you take over.
You are in no way required to take over your mother’s dangerous abandoned project horse. Just stop working with this horse before you get seriously hurt.
You don’t have the legal standing (age or ownership) the money or the autonomy to seek out help or make decisions about this horse.
The only decision you can make is to stay safe.
I also think you need to find another trusted adult to talk with since I don’t think your mother sounds like she is in the headspace to make safe decisions for you. Father, aunt, uncle, riding coach, teacher, school counselor etc.
Ok, I just read the whole thread with the extra information about the family dynamics. It’s unfortunate that you are still on the same property as your mother even after CPS intervened.
Your grandmother is your guardian I assume, so crazy threats from your mother shouldn’t have any reality. This is a really hard place to be in, and the horse is just the visible tip of the chaos.
You need to keep yourself safe and stop interacting with the feral horse.