A horse doesn't have to pull a long face, it has one already. But it can and does look down its nose at you. It can be insistent, if not a nag. Horses are expressive, but those who want to decipher their expressions have a few hurdles yet to jump.
Scientists at the University of Sussex have compiled a directory of facial expression in one of humanity's oldest four-legged friends. Their Equine Facial Action Coding System has identified 17 discrete facial movements in horses that may indicate mood or intention or just bafflement.