From Wikipedia:
Grullo (also black dun, blue dun or lobo dun) is a color of horses in the dun family, characterized by smoky or mouse-colored hairs on the body, often with shoulder and dorsal stripes and black barring on the lower legs. In this coloration each individual hair is mouse-colored, unlike a roan which is composed of a mixture of dark and light hairs. In the Fjord horse breed, the grullo color is called "grey," or, in Norwegian,"grå" or "gråblakk" (literally, "gray dun"). However, a grullo is not a gray horse and carries no graying gene. It stays the same basic color from birth (with seasonal shade variation from summer to winter coats in some individuals) and does not progressively lighten with age.

The word grullo originates from the Spanish word "grulla", which refers to a slate-gray crane. Because of the Spanish origin of the name, some people will refer to a mare as a grulla and a stallion as a grullo, pronounced "grew-ya" and "grew-yo" respectively.

A grullo horse is a dun with a black base, rarer than red duns or bay (classic) duns. Only 0.7% of quarter horses registered each year with the AQHA are grullo. There are several variations of grullo, informally referred to as black dun, blue dun, slate grullo, silver grullo, silver dun, or lobo dun. However, all of these shades are genetically based on the same thing; the dun gene on top of a black gene.

The most obvious way to tell a grullo is not only the gray or tan-gray shading, but also its primitive markings, which include some or all of the following: dark face; cobwebbing around the eyes and forehead; dark mottling on the body; leg barring (sometimes called tiger striping); dark ear tips and edging; dark ear barring; dark shadowing of the neck; dark dorsal and transverse striping; dark mane and tail guard hairs.
Grullo is one of the many colors of the Icelandic horse.

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