The OP does have a contract, just not a written one. Working out what its terms are from Facebook is another matter though.
The OP might want to mitigate her losses by trying to return the horse, but if the contract of loan is really for 12 months and the owner requested she insure the horse for vet's bills, then I cannot see why the OP as loaner can avoid being responsible for 12 months care of the horse and vet bills incurred during her care of it. I'd also be aggrieved as owner if the horse had developed a medical condition while in the care of the loaner, and one which is relatively common and easily treated but is not being.
Most insurance companies don't even specify vettings for horses insured for less than £2000 including vets' bills, and simply specify passport number for ID.
Any horse, whether you buy it or loan it, may develop lameness or injury which might prevent riding for a while.