They were accidentaly created and grew as bipedal herbivores, they mostly ate apples. eventually Mojang tried to delete them but they could not reach on area,CAVES, eventually most of the species died not knowing that. As cave dwellers it was difficult to survive they could only eat mushrooms and they were rare so they died of starvation. As they ran deeper and deeper they found Rubies! so they just just ate them thinking they were apples, this alterated their bodies pretty fast making the Rubies grow on their bodies and from all the time away from the sunlight they're skin lost it's color, not all could eat the rubys so there are very few of them around... They are also the reason Rubies never went in the game and were substitued by Emeralds