I found my best horses in the herd that was following the big black stallion around, in the Taobob Grassland. All 4 horses I tamed there had 5 in at least one stat, usually rounding up to 13 points altogether. One of the horses was 5 strength 5 speed. So fast you don't even need those 2 extra stamina, but you can feed it a carrot for more if you really want to. My current horse is 2 strength, 5 stamina and 5 speed.