The loan agreement for loaning a horse is on the BHS website and is something you can download and then edit as you need too. I have one for Trojan and both myself and his Mum put in the bits we wanted and then it was signed by myself, her and 2 witnesses.
Please anyone do not consider loaning a horse without this agreement, you do not have a leg to stand on without it either as the loanee or the loaner.