Ok -- so I'm back but I only have a few minutes before work...
To pick up... I have found that the vital elements of fundraisers are: volunteers, legitimacy of fund, people must feel like they are getting something for their money -- a prize, their name posted, updates on animals/project/fund, photos, etc. That's where the donations become so important.
Making the community aware of what the rescue is doing before starting up the fundraiser is important maybe through flyers, radio interviews, community newsletters. Having a particular project or goal that you can keep the locals updated on helps. They like to see what's being accomplished. Maybe the local paper will run a small credit weekly or monthly for you.
Go over the past fundraisers and see if you can dig up some comments from people that helped, that didn't help -- why? If they aren't a legal not-for-profit, it makes it harder. Perhaps a local bank will donate a fee-free account that is solely for the horses. People don't want their money to be used for the owner's grocery bill, they want assurance it's going to the animals.
Hope something here helps. Good luck.