So once you've made a mistake that you think will last forever think again!
Simply go to the horse breeding market (accesed via the stables), go to my registrations, find that dumb horse aaaaaand cancel!

First go to the stables and click on the male you would like to breed, click register for breeding, now before you confirm check the box that says mine or something like that if you don't want it to breed with horses other than your own, next in the stables click breeding market, if you want to breed with your own horses go to my registrations, now click breed on the perfect horse and select your female that you would like to breed, and you're done!

If you can't buy a saddle than i'll tell you how! The most likely problem you're having is that you're not friends with the stable worker, yup that's right it costs amity to buy certain items, that's okay simply converse with them until you have enough amity. To equip a saddle stand near your horse and right click the item in your inventory that you would like to equip.

Hope this guide helped you with your horse s***.
Thanks for reading! Have a nice grinding session!