Once upon a time, I had designated a person to take care of two of the four, with the other two getting PTS'd.
Enough years have passed and the youngest (19) has severe insulin resistance and, has foundered, is frustrating & and expensive to manage, so now they all would get PTS'd if something happens to me.
My horses are 19, 20, coming 27, & 28 and I spend a bloody fortune on all of them. They would go downhill fast in 99% of even the most experienced horse owner's hands because nobody wants than kind of expense.
Just because "it is so written" in the Will, does not guarantee your wishes will be carried out, where the horses are concerned. It costs a lot of money to PTS a horse, then get the back hoe out to lay it to rest.
My vet, the farriers, my horse neighbor, and an old trail riding friend all know that somebody had better get my horses put down "five minutes ago", if something happens to me and my husband doesn't take action. He says he will but he's also a coward when it comes to PTS'ng a dog, even when he clearly understands it's the more humane thing for the dog.
Whatever arrangements you make today, be sure to re-evaluate those arrangements every five years. It is amazing how situations and friendships can change (for better or worse) in that small time frame