The Agouti is actually a large South American rodent, why in the world this gene was named after it is beyond me! There are two pigments involved in producing a colored coat, red and black. The Agouti gene determines exactly where in the animal's coat the black pigment appears. In horses, the Agouti gene allows black to appear on the legs, mane, and tail, and not on the body. Other genes determine whether or not the animal actually produces black pigment at all, which explains how an animal can carry the Agouti gene and not show it.