noun
A specific term used in horse breeding and equestrian contexts to describe a dark brown coat color, often with a black mane and tail.
あの馬は青鹿毛で、とても美しい毛並みをしている。
That horse is a brown with a beautiful coat.
栗毛 is a chestnut coat color, lighter and more reddish than 青鹿毛.
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.