I've copied this from another thread on another website, as this mod had been done before but it's currently outdated with no signs of an update. This original post describes better than how I would be able to describe how a mod like this would work.
"I want to change the horse breeding formula. With the current fomula it is really hard to improve your horse's stats with breeding, so the only way to get better horses is to find them. I want to change the formula so that it's possible to improve your horse's stats over generations, but still not easy or cheap!
I would change it to be the average of the two parents and then a random -5% to a +5% bonus taken away from or added to that average.
Parent 1: 12,36 m/s
Parent 2 13,01 m/s