Rarity and Parts of Derby Horses
First of all, it is very simple how common and rare grades are decided among the three grades of horses. If at least one of the eight parts of a horse has a Rare rating, it is a Rare, and if not, it is a Common. Common is effectively treated as Rare 0, and as you already know, Rare horses exist from Rare 1 to 8. This is because Derby Stars horses have a total of 8 parts.