What is a Grullo? A grullo/grulla colored horse is a black horse with a dun gene. The dun gene dilutes the horses black coat color to a “mousy” light, medium or dark slate color. The mane and tail remain black. This unique color is also characterized by primitive markings that include a dorsal stripe, zebra like leg barring, dark face and dark ear tips. Grullo is a color, not a breed! All the horses in this reel are Quarter Horses registered with AQHA in our program. Bonus points if you can tell