Irishman Daniel Sullivan, who died in 1810, was a horse trainer and rehabilitator. Not much is known about him, since he was secretive about his actual methods. Some accounts say that he learned his basic method from "a gypsy". People who had seen him work assumed he was whispering to the horse because he stood so close to it. For that reason he became known as the "horse whisperer."
There are several well-known horse whisperers - or people using natural horsemanship methods to great effect - including Monty Roberts, Pat Parelli and Mark Rashid. Their methods vary, but they commonly use horse psychology and body language to train horses in a non-abusive manner.