saddle mark
/ˈsadl mɑːk/noun
1A mark on a horse's back resulting from the friction of a saddle, often showing as a whitish patch of hair or skin.
2An area of hair left unclipped on a horse's back where the saddle is placed.
3A natural saddle-like marking on the back of an animal.
Origin
Late 17th century; earliest use found in The London Gazette. From saddle + mark.