As others have pointed out, if you haul horses for money, you need commercial insurance as you are clearly responsible for the animals you are hauling. An insurance agent can help you sort out the options. Hauling horses for a friend with them chipping in for gas does not make you a commercial hauler, but also the fact of not charging money does not absolve you of responsibility should an accident occur.
Even just hauling horses casually for friends, I still strongly recommend having a conversation with your insurance agent about how they would handle the situation if something happened while you were hauling a friend’s horse. Just because you aren’t commercial and aren’t charging money doesn’t mean that people can’t hold you financially responsible and expect you to “make things right” or even sue you if something should happen.
Let’s say you glance at your phone and get in an accident or have to slam on your brakes and the horses are injured–even though you were doing your friend a favor, the friend might perceive that the accident was your fault and thus feel you are responsible for vet bills. So many things can happen–a mechanical issue like a sharp edge from a backed out rivet and the horse gets a horrible cut or a horse or a person gets kicked or stepped on or run over when loading or unloading.
An insurance agent could hep you sort out what insurance would best cover you. It’s possible an umbrella policy or horse owners liability policy would cover you for any accidents where humans were hurt. I think covering the horses is a lot more difficult. It’s very difficult to insure against being held responsible for injury/loss of other people’s property/livestock, unless you have a CC&C policy.
I run a boarding facility and have a commercial policy and a CC&C policy that covers me for hauling horses that I am responsible for as part of my farm business only. I’m sure this type of policy exists in other states, that is why you see trainers/farm owners able to haul their clients horses around without separate commercial hauling policies.
In making your final decision, it’s also helpful to know what the equine liability landscape looks like in your state. Some states have laws related to activities related to farms and livestock that help protect farm owners and horse owners from liability. What assets you have is another factor in making a decision about insurance. If you have a lot of assets, it makes sense to be more conservative about insurance, or even hauling other people’s horses at all (hauling horses is a very high risk activity).