A specific term used in horse breeding and equestrian contexts to describe a dark brown coat color, often with a black mane and tail.
That horse is a brown with a beautiful coat.
Compound of 青 (ao, 'blue/green', here used for a dark shade) and 鹿毛 (kage, 'bay horse color'). The exact historical derivation is uncertain, but the term is conventionally used for this specific coat color.