For these exceptional situations, I think you have to make a Half Board contract, because the criteria that are stipulated in it (who pays what, who has the right to do what on the ride, when on what days to ride it , Exit in authorized walk ...). These are things that need to be clarified in order to allow each of the two parties to be reassured. After all, I think that good agreement between the two parties is essential. I think that a contract for a given horse must be more precise than that of a half-board, for the simple reason that when you give a horse there is no "return" possible, As a commitment without any real counterpart. In addition, as time goes on, it will be necessary to add contracts to the contract. We can not plan everything from the start ... That's why communication is essential.
A tip, whenever there is a change of contract or an addition, it will have to be written !! To have a contract as complete as possible and not have any problem :)