I have a weakness for this kind of niche simulations. The ones that illustrate a few special processes with maths without unnecessarily adding a lot of side aspects.
The layout could be reworked a bit. Packing the trainings into a table, for example. And a few graphics could also loosen things up.
So there are a few errors:
Under trainings: "Speed IV+: Min Level" (the plus)
under trainings point: "Levels 0, 1, 2, 3, 4, Level 4 to 5; Requires 15 Training"
Also under Trainings points: "Are considered low High Level" or did you mean something like semi profi level?
But all in all i really like it.
P.s.: I haven't done the math yet, but wouldn't it be very easy to make money by buying a horse that has level 5 on many stats (500$ per stat), train it by one level and sell it again (per stat on level 6 is 3000$)? That sounds like a huge profit margin.