There is also another idea and it's already working in my head...
"Thoroughbreds are based on selections, breeding and then again selection, breeding and so on."
I'm thinking about a generation of generated and simulated horses as foundation and then include an additional value based on their races and wins (only top 10 and top 3) and their experience. This value increases during the time and can then be used to breed. With an additional dice table, you can then try to create new horses and they will get some additional (specific) values, based on their parents, that increases (maybe) their strength, run, jump or other values.
This also means, that you can breed horses, that can deal better with jumps, other ones with long distances, heat and so on. This will add another, interesting part to the game and the fun for long-term runs. But there will be still a random impact to the breeding table, to avoid to much development, which would create strange horses and so there will be breeding lines, that will fail after a while. For example a horse, that has too much strength or power, will maybe suffering from medical troubles or failures. So high end horses will come with a price too.
I will add this experimental part in one of the following updates.