In many cases the technical legality of a situation is fairly irrelevant, particularly where livestock are involved.
What exactly are you worried about? That your boar/pigs come back in a terrible state - skin and bones, hobbling on 3 legs and full of worms and lice, and that the RSPCA take you to court as the owner and not the person who has borrowed him or was looking after them?
Or are you worried that the animals come back in a similar state and you want to be able to to sue the person who was keeping/looking after them?
Either way, surely the important thing is that you wouldn't let an animal of yours go to anyone unless you were absolutely sure in your own mind that he was going to be well looked after. Liability doesn't really come into it, but common sense does. Whatever the law says, if you've taken all obvious precautions to ensure his proper care , then there's no way you can be taken to court for ill treatment obviously caused by another person.
If you're worried what comeback you would have if the boar/pigs were returned in a poor condition- then in reality - very little, unless the keeper agreed to pay you compensation, which is unlikely. So you would then have to take them to court, get a vet's report, prove the boar/pigs were fit and healthy when they left you (which the lender could deny) So would you really bother?
So basically if all these things are of concern to you, then is it worth the risk? The legal system does not work as a sort of insurance to guarantee responsibility, it's really down to commonsense.