The OP wasn't told that the horse had been deemed unrideable when she bought it - that is something she's found out since.
She was lied too by the seller who knew the horse has issues but didn't disclose them - and lots of things don't show up on PPE's so sometimes having one won't help
I've seen videos of the OP riding this horse when she first began working her and she was doing really well with her - she's a very good young horsewoman -no doubt some old injury has resurfaced to cause the problems that have now arisen
At least she's doing the responsible thing and not selling her on
Don't get this post
- clearly the mare is ridable as the OP has videos of herself riding the mare (wrapped in a tarp, no less). Mare is sound in the video, looks comfortable at the walk and the trot, and is listening to her handler. I see no signs of unsoundness, either physical or mental.
PPE is the responsibility of the prospective buyer - she didn't get one so this is irrelevant. Any speculation on an old injury is just that, SPECULATION.
A "very good young horsewoman" (VGYH) doesn't usually buy a horse after being led around on it bareback while it wears a rope halter. A VGYH doesn't usually buy a horse, knowing it's a project, without a PPE or having it looked over by a trainer, then hop on wearing a tarp - then, when it rears, sometime later, decide its unridable and should be put down, AFTER said VGYH goes on public forums saying she's going to ruin the seller's reputation, sue her, and make her life miserable.
Add to this she's owned the horse for at least five months, and has posted questionable "bonding" videos, which - If I was the seller - would point out that any bad behavior owner has experienced could well be due to the handling practices of current owner.
Last, any horse can be dangerous. Any horse can have a rearing episode, or buck, given the right circumstances. It doesn't mean the horse cannot have a good future, be resold, or should be condemned to die. This VGYH was warned, by the seller, from the beginning, she was not a good fit for this mare. She bought it anyway. OP doesn't have a defensible postition.