As somebody who grew up with horses and had some who were 'special', too, you have my deepest empathy.

Would the barn owner be willing to accept partial payment in trade, such as work around the stable?

This is so sad.

I don't know how anybody can 'cast off' a horse either.

The horses of my youth were my best friends, along with my dogs and were always there for me when my own family 'didn't have the time' or concern.

I'm sorry I have no better advice to give but you do have my sympathy.