In theory most businesses won't turn away a volunteer. In fact, many will specifically offer vacancies for volunteers.
Volunteers are great for businesses as they do not require a wage and are the foundation of the work force. They often have experience in the job they seek, and they are often flexible with time.
As I said, most businesses won't turn you down, but I was turned down once. The reason? I wasn't in the right area or borough. The charity wanted to give any opportunities they had to those living in the borough where they were based. I think that this was a little unfair. Volunteers can be hard to find and you could be waiting absolutely ages before one comes along. Grab them whilst you can!
Sometimes, if you've worked for pay for many years, it is hard to get into the mindset of the volunteer. You are not obligated to do anything you don't want to, and, if you require time out, you can take it. After all, you are there because you offered the help. You are not paid to be there. So, if you want to spend a little quality time with the horses or animals, you can and you shouldn't feel guilty doing so.