Babysitting is generally yer best bet for making useful money.
And/or a paper route if you live somewhere sufficiently suburban for such things to exist nearby.
Braiding works too *if* your parents are willing to drive you around in the wee hours of the morning, but unless you are quite skilled AND fast at it, AND happen to live in an area where there is high demand and low supply of braiders, I would not necessarily count on a *lot* of income from it.
Don't even think about trying to get paid for riding horses at this point.
For one thing if you are seriously selling Toby to buy a foal, for some unfathomable reason, as opposed to merely floating this idea because it appeals to you this morning, you are going to need alllllll the professionally-supervised horse handling experience you can get.
Or, you could make FAR more efficient use of your money by NOT buying a foal, either keeping Toby or buying something you can actually RIDE NOW rather than having to wait 2-3 years before you can even get on (and then hoping nothing bad has happened to the animal during that time, like becoming unrideable or dead).
Good luck, have fun,
Pat