When I loaned a cob, I paid for everthing except the insurance and the owner cancelled that and didn't tell me, so had to go halves when he was put down. And it was in a written contract. If you take a loan horse you should pay for everything, except insurance. But to be honest just buy if you can afford.
You normally pay for everything except the cost of buying the horse. Some owners will want you to pay for the insurance, some owners will like to pay for the insurance themselves it depends. My first mare I paid the insurance, my 2nd loan the owner did.
As you pay for everything it is still as expensive as owning usually. You could also look into getting a share which can be free/paid (£10-£30 a week) or some owners are just happy for you to help out with the mucking out etc.