What is it called when you shave the horse's mane right behind the ears?
The bare spot where you shave a horse behind the ears is called
a "bridle path". It has that name because it clears a path on the
horse's neck for the bridle to be worn without getting tangled in
the mane. There are different styles of bridle paths, from only
cutting a tiny bare spot, to shaving the mane from ears to half-way
down the neck. When the entire mane is shaved off, the mane is
"roached."
So when you shave the horse's mane right behind the ears, it's
called "cutting a bridle path." And when you shave the entire mane
off, it's called "roaching the mane."