The fairy tales of taking on the impossible horse, and turning him to a star citizen, maybe a blue-ribbon winner, are for the young adult books.
This horse desperately needs the right home, and OP has a heart of gold to try to help him find one. It needs to be a home that is ok if he doesn’t have the fairy tale ending. If there just isn’t anyone who has the right skills to put him on the right path to be a happy worker. A home that will care for and treasure him even if he’s just a pasture ornament for the next decade or so, maybe an expensive one.
I pray this horse finds that home. With the OP’s help he certainly has a better chance than if everyone gave up on him. But if it ends in euthanasia, I can support that, just out of a realistic assessment of his chances in the world as it is today. There are not enough homes for pasture ornaments. And even fewer people who have the skills, perseverance and wisdom this horse needs to actually correct his problems. It could be very expensive and time-consuming because he very probably does have physical issues affecting his behavior that have to be corrected before he can be a reliable riding horse.
Hope we get to hear the ending someday. Best of luck to this horse!