You could groom the horse and make it a race horse after much practice, you could take it to a ranch where it will have all the proper care it needs, you could also take it to a circus or a park where the horse can be used for riding. Mostly though I think if you have a horse take it to the countryside and breed more which you can either sell or create a horse ranch with.
Either take care of it yourself or give it to someone that knows how to take care of it. Also, have a consultation with the vet to make sure its vaccines are up to date and its healthy and it can be ridden and everything. Be careful, those bills can be big.