You can do chores for your neighbors (pet sitting, running to do jobs for them etc) if you go to a barn where you take lessons at you can come early and muck out a few stalls for a bit of money, you can sell old clothes or riding things that you don’t need anymore. Hope this helped!
Many places you won’t be able to work, but you can find some off the table jobs. Babysitting is always a good job if you can get it, it pays well usually and it can be easy. Lawn mowing, doing chores for neighbors, that type of thing. I know where I live there are a few farms that you can work at when you are 15. You can also work at Shaw’s where I live if you are 15. Good luck!