If you owned the horse it would be your responsibility. When you loan you may not always be liable, it depends on what your loan contract says (some people insure their horse before loaning it, others leave that up to you) and what insurance you have. You should make sure you yourself are covered and that your horse is covered for injury/damage to others at the least (third-party). Insurance doesn't always come cheap, and you've gotta do your research before you decide exactly what cover you want, but it is worth it if anything like that should happen.

Before my horse, I had one on loan, and it was a fantastic experiance! If you can't afford your own horse, then loaning is a perfect alternative. Like when you'd be buying a horse, be picky, don't go with one just because it's the only one around, wait for the one for you. Also, make sure you definatley can look after a horse, and know exactly what to do (Maybe a trial at your yard?)if you are confident with it all, then go for it. It's brilliant and enjoy it whilst you can!