Something I’ve seen work well for human children guardianship is to have a designated guardian (caretaker) and a financial fiduciary (trustee). So there’s a system of checks and balances- funds can’t be released by the trustee (financial fiduciary) without the backing of the guardian.
The beauty of a trust is you can designate the terms of it as you wish, in line with current estate laws.
Likely the least complicated way would be to leave a letter of directive of approved farms and if all else fails, someone you trust who could choose another option. Aka- Dobbin can go to farm a, b, or c. If none of the above are accepting new horses, in existence, etc. knowledgeable friend X may choose an alternative. Requirements include….etc etc.
ETA- the directive letter is what we did. The letter can be updated periodically and is kept with the will/estate plan. So there’s the formal trust then the letter to the trustees on where the horses should go and who to call if those are not viable options. In my case, the trustees are not horse people so my horsey friend is their lifeline to evaluating retirement homes, arranging transport, etc.