The Dragons have 3 Available Rarity up to now. They are included: Common, Uncommon, Rare. The Rarity affects so much to the Dragon's strength. It will limit the number of Dragon Ability's Ratio, Number of Skill slots of that Dragon,... And with the Higher Rarity of the Dragon, the higher stats come from the Dragon Parts to give a powerful base stat of the Dragon than others.
The higher of Rarity of Dragon, the larger number of Dragon Skill slots
The higher of Rarity of Dragon, the larger Chance of getting stronger Ability