You need to do something that fits your talents and skills. if you are detail oriented and good at numbers, a self-starter, and don’t mind working alone, accounting would be a good fit. If you are more of a people person, and have good interpersonal skills, and are bored by too much time staring at the computer screen, then you would probably be a better fit on the management side of things. If you have an analytical, academic brain, plus are very good at math and statistics, you might be a good fit for economics, which is much more academic and research based than accounting or management. To a large extent, you may not really know until you get into college and try out different courses.
I would say: go to the highest-rated college you can get into, and can afford. That will increase your employability afterwards. And get good grades, get summer jobs in your field if you can, do everything to make yourself as employable as possible right out of school.
As far as working with horses, the thing to keep in mind is that let’s say you start a successful small business doing the books for several big barns in your area. You still aren’t riding! You are just adding up other people’s money. Let’s say you go to work for the race track or a big breeding barn. You still aren’t anywhere near the horses. And horses, being part of the agricultural sector, tend to pay less than other fields, and being a very simple business, tend to have far fewer opportunities for advancement.
So you might be best off going for the best field you can find in terms of pay and advancement, and then using your salary to buy and board a nice horse.
Also think about where you want to end up living. Some businesses are clustered in certain parts of the country, some clustered in big cities. Few are in truly remote rural areas. Where do you want to live, for horses but also to be close to family (or far away from them, whichever suits you)? For instance, the oil and gas industry could take you to Texas, which has horses. Working for the stock market, on the other hand, would put you in downtown Manhattan, a long commute away from horses. You might be able to get into middle management at some retail chain that would let you move out to smaller cities.